8.1 understanding acids and bases - quia

220 Unit 3 Solutions and Solubility Copyright © 2002 Nelson Thomson Learning

8.1 UNDERSTANDING ACIDS AND BASES

PRACTICE

(Page 367)

Understanding Concepts1. Ionic compounds when melted will conduct a current (which suggests that they contain charged particles such as

ions), while molecular compounds in the liquid state will not (which suggests the absence of charged particles).2. (a) acidic

(b) basic(c) neutral(d) acidic(e) neutral(f) basic

3. Acids are unlike other molecular compounds in that their aqueous solutions conduct a current, turn blue litmus red,and neutralize bases.

4. According to the Arrhenius theory, litmus colour change in acidic solution is caused by H+(aq) ions, and litmus colourchange in basic solution is caused by OH–(aq) ions.

5. Empirically, an acid is a substance that, in solution, will turn blue litmus pink, conduct a current, and neutralize a base.Theoretically, an acid is a substance that will release H+(aq) ions in solution.

6. (a) NaOH(s) → Na+(aq) � OH–

(aq) (dissociation)

(b) HC2H3O2(l) → H+(aq) � C2H3O–

2(aq) (ionization)

(c) H2SO4(l) → H+(aq) � HSO–

4(aq) (ionization)

(d) Ca(OH)2(s) → Ca2+(aq) � 2 OH–

(aq) (dissociation)

Note: Acid–base concepts in this text do not address the ionization of polyprotic acids such as sulfuric acid. Singleionization is assumed until later in the student’s course of study. (See the example with oxalic acid, H2C2O4(aq), shownon page 364.)

Applying Inquiry Skills

7. (a) AnalysisChemical 1 is the molecular compound, C12H22O11(s), (sucrose) because it dissolves well, to produce a noncon-ducting, neutral solution.Chemical 2 is the ionic compound, KCl(s), because it dissolves well, to produce a conducting, neutral solution.Chemical 4 is the ionic hydroxide, Ba(OH)2(s), because it dissolves well, to produce a conducting, basic solution.Chemical 7 is the acidic molecular compound benzoic, HC7H5O2(s), because it dissolves somewhat, to produce aslightly conducting acidic solution.Chemicals 3, 5, and 6 are zinc, calcium phosphate, and paraffin wax. They are not individually identifiable from thisevidence. Because none of them dissolve, no observations of the solutions can be made.

(b) EvaluationThe experimental design could be improved by testing the conductivity of the pure substance (which would identifythe metal zinc), and by testing the melting points, which would distinguish ionic calcium phosphate (very high) frommolecular paraffin (fairly low).

Making Connections8. Both acids and bases make good solutions for different types of batteries. Lead–acid and alkaline batteries are the

most common examples.9. Acids (a), (c), (e), and (f) are used for energetic reactions and should be handled with care. Acid (b), carbonic acid, is

a weak acid that has low solubility, so it is not dangerous. In fact, it is an ingredient in most soft drinks. Acid (d),acetic acid, is dangerous in pure form, but in vinegar is only about a 5% solution so is not very dangerous because thesolution is so dilute.

10. You can be electrocuted anywhere, if current has separate places to enter and to leave your body. If the pure water thatyou are standing in is part of the circuit, it will be a very poor conductor, and you should be relatively safe. In fact,

dissolved substances (for example, from your boots or feet) would make the water in such a situation a relatively goodconductor. It is always best to assume that electricity is dangerous around water!

8.2 pH OF A SOLUTION

PRACTICE

(Page 371)

Understanding Concepts1. Examples of products with specified pH values include skin creams, soils, shampoos, cat foods, wines, water (in rain,

wells, lakes, rivers, aquariums), and waste water outflows.2. [H+

(aq)] values are:(a) 1 � 10–11 mol/L(b) 1 � 10–2 mol/L(c) 1.0 � 10–4 mol/L(d) 1.0 � 10–14 mol/L

3. pH values are:(a) 3.0(b) 5.0(c) 7.00(d) 10.00

4. [H+(aq)] values would have to change from 1 � 10–5 mol/L to 1 � 10–7 mol/L: a ratio of 100:1. The hydrogen ion

concentration would have to decrease by a factor of 1/100.5. Since log 1 � 100 exactly, by the definition of logarithms, the pH of a solution with a hydrogen ion concentration of

1 mol/L will be 0.0.Note: Since the concentration is given to one significant digit, the pH is reported to one decimal place (one place

in the logarithm characteristic).6. (a) vH+ � 100 L

CH+ � 1 � 10–3 mol/L

nH+ � 100 L� � �1 � 1

10L�

–3 mol�

nH+ � 0.1 mol

The amount of hydrogen ion present in the wine is 0.1 mol.(b) vH+ � 100 L

pH � 8.00

CH+ � 1.0 � 10–8 mol/L

nH+ � 100 L� � �1.0 �

110

L�

–8 mol�

nH+ � 1.0 � 10–6 mol � 1.0 mmol

The amount of hydrogen ion present in the seawater is 1.0 mmol.(c) vH+ � 100 L

CH+ � 10.0 mmol/L � 0.0100 mol/L

nH+ � 100 L� � �0.01

100

L�mol

�

nH+ � 1.00 mol

The amount of hydrogen ion present in the stomach acid is 1.00 mol.

Reflecting7. (a) No, setting a zero measured level would have to specify the precision of the zero measurement. A more precise

technology may turn an earlier measurement of zero into a non-zero value. Secondly, every chemical is toxic atsome concentration. There is no known chemical that needs to be reduced in concentration to zero (whatever thatmeans) in order to become non-toxic.

Copyright © 2002 Nelson Thomson Learning Chapter 8 Acids and Bases 221

(b) Theoretically, a zero level would mean no entities (atoms, molecules, and/or ions) of the chemical present at all.Empirically, there would be no way of confirming or refuting a theoretical level of zero. Although a theoreticallevel of zero is possible, it is not probable, and, again, is impossible to test.

(c) A zero level is not measurable; a zero reading may always be taken to mean that the quantity of chemical is justtoo low to be detected by the instrument. A measurable zero quantity is a contradiction in terms: it would requirea perfect measuring device and/or system.

(d) One possible answer: A limit should be set low enough so that there are no measurable effects from such a level,even over a lengthy period of time. Note: How long a time? Ahhh, now there’s a good point for discussion and opinion. ... The world becomes a

very much trickier place the moment students wrap their minds around the concept that there is no such thing as“perfectly” safe.

PRACTICE

(74)

Understanding Concepts8. Solution pH is measured with pH paper, with indicators, and with pH meters.9. (a) Pure water has a pH of 7.

(b) The hydrogen ion concentration of pure water is 10–7 mol/L.10. (a) The calculation of values to complete spaces in Table 2 are, in order:

OrangespH � –log [H+

(aq)]

� –log [5.5 � 10–3 mol/L]

pH � 2.26

Asparagus[H+

(aq)] � 10–pH

� 10–8.4

[H+(aq)] � 4 � 10–9 mol/L

Olives[H+

(aq)] � 10–pH

� 10–3.34

[H+(aq)] � 4.6 � 10–4 mol/L

Blackberries

pH 5 –log [H+(aq)]

5 –log [4 3 10–4 mol/L]

pH 5 3.4

(b) Based on pH values only, oranges are the most acidic, and should have the most sour taste.

Making Connections11. Answers should always present both sides of the risk/benefit concept. An example might be the use of acidic cleaners

to remove rust stains from plumbing fixtures. The aesthetic benefit of shiny plumbing fixtures carries with it the riskof skin irritation or disfiguring marking of other surfaces, resulting from careless use of the acidic cleaner; anddisposal might be a problem if the cleaner is not greatly diluted.

SECTION 8.1–8.2 QUESTIONS

(Page 375)

Understanding Concepts1. (a) [H+

(aq)] � 1 �10–3 mol/L for the fruit juice, and 1 �10–12 mol/L for the household cleaner.


(b) The hydrogen ion concentration ratio, juice to cleaner, is

�11

�

�

1100–

–

1

3

2mm

ooll//L�L�

�� 1 �

1109� , or a billion to one.

2. (a) pH � 8.0

(b) pH � 7.0

(c) pH � –log [H+(aq)]

� –log [2.5 � 10–6 mol/L]

pH � 5.60

(d) pH � –log [H+(aq)]

� –log [1.3 � 10–4 mol/L]

pH � 3.89

3. (a) Pickling vinegar is more acidic.(b) Pickling vinegar has a higher hydrogen ion concentration.(c) [H+

(aq)] � 10–pH

� 10–2.4 mol/L

[H+(aq)] � 4 � 10–3 mol/L (standard vinegar)

[H+(aq)] � 10–pH

� 10–2.2 mol/L

[H+(aq)] � 6 � 10–3 mol/L (pickling vinegar)

4. (a) The hydrochloric acid has a lower pH because the molecules ionize to a much greater extent than the acetic acid.>99%

(b) HCl(aq) → H+(aq) � Cl–(aq)

<50%

HC2H3O2(aq) → H+(aq) � C2H3O–

2(aq)

(c) The HCl(aq) should be handled more carefully because it will react much faster, and so is more dangerous.

5. (a) [H+(aq)] � 10–10 mol/L (pH paper)

Note: When a pH value has no digits in the characteristic at all, it represents a quantity that is known only to anorder of magnitude—that is, a quantity with no significant digits; where only the place value is known. This level ofcertainty is fairly common when pH is measured with coarse-scale pH papers. Measured values with no significantdigits, or even those with only one significant digit, are rare. They seldom occur outside of systems like pH, that havesuch a great range that logarithmic scales must be used to express them.(b) [H+

(aq)] � 10–pH

� 10–9.8 mol/L

[H+(aq)] � 2 � 10–10 mol/L (pH paper or pH meter)

(c) [H+(aq)] � 10–pH

� 10–9.84 mol/L

[H+(aq)] � 1.4 � 10–10 mol/L (pH meter)

(d) [H+(aq)] � 10–pH

� 10–9.836 mol/L

[H+(aq)] � 1.46 � 10–10 mol/L (very precise pH meter)


6. (a) [H+(aq)] � 10–pH

� 10–5.4 mol/L

[H+(aq)] � 4 � 10–6 mol/L

(b) [H+(aq)] � 10–pH

� 10–5.72 mol/L

[H+(aq)] � 1.9 � 10–6 mol/L

(c) pH � –log [H+(aq)]

� –log [5 � 10–7 mol/L]

pH � 6.3

(d) pH � –log [H+(aq)]

� –log [7.9 � 10–6 mol/L]

pH � 5.10


7. Experimental DesignEach of six samples of solutions of different acids, of equal concentrations, will be tested for conductivity, to deter-mine which are strong acids.Note: Alternatively, pH paper, indicators, or a pH meter could also be used.

8. PredictionSince a change in hydrogen ion concentration of 100 times (or 102) represents two pH units, it seems probable thatwhen 1.0 mL of vinegar is diluted 100 times, the pH (characteristic) will increase by 2.

Experimental DesignThe pH of some 5% V/V household vinegar is measured to one place in the mantissa (i.e., one decimal place). A 1.0-mL sample of the household vinegar is diluted to 100 mL, and the pH of the diluted sample is measured.

Materials• household vinegar (5% V/V acetic acid)

• wash bottle of pure water

• 10-mL graduated cylinder

• 250-mL graduated beaker

• stirring rod

• pH meter (or pH paper, precise to 0.1 unit )

Procedure1. Measure and record the pH of some household vinegar.2. Obtain 1.0 mL of the vinegar in the 10-mL graduated cylinder.3. Use a wash bottle to rinse the vinegar sample into the beaker.4. Add pure water to the beaker until the solution volume is 100 mL.5. Measure and record the pH of the diluted vinegar.

EvidenceThe pH of the initial vinegar sample is 2.4.The pH of the diluted vinegar sample is 3.4.

AnalysisThe volume to which 10 mL of vinegar must be diluted in order to increase its pH by two units is more than 1000 mL.The pH value only changed by about 1, indicating that although the acid solution concentration was decreased 100times, the hydrogen ion concentration seems to have decreased only about 1 � 101, or 10 times.


EvaluationThe experimental design is judged to be adequate because the experiment produces the evidence needed to answer the(very simple) question with a high degree of certainty (even though the Prediction was found to be incorrect).

The procedure is also judged adequate because the steps are simple and clear.The technological skills of the experimenter are judged adequate — no special skills are required, and the

procedure is very straightforward.The results have a very high degree of certainty. None of the materials or procedure steps are likely to have any

unexpected uncertainty. The percentage difference cannot be calculated as such, but the answer differs from theprediction by at least an order of magnitude.

The prediction is definitively falsified because the evidence is clearly different from the prediction.The assumption (that the pH value varies proportionally with the negative of the logarithm of the concentration

of a weak acid) is not supported by the results of this experiment. It seems likely that there is a more complex rela-tionship between weak acid concentration and hydrogen ion concentration than was assumed.

Note: This is a nice example of a case where a simple concept turns out to be more complex — offering an oppor-tunity to have students learn by falsifying a prediction. Students’ only experience with dilution and pH to this point iswith the strong acid HCl(aq) (Investigation 8.2.1,) and from calculations involving strong acids. Students will normallyform predictions based upon the pH � –log[H+

(aq)] equation, or upon their experience from Investigation 8.2.1. Neitherof these methods is useful when dealing with a weak acid, where the varying degree of ionization is a predominantfactor. Empirically (experimentally), the final volume (100 L or more) is required to change the vinegar concentrationby two pH units. A theoretical explanation comes in Chemistry 12.

9. The design is probably valid because toothpaste is diluted in your mouth as you brush, anyway — so the pH meas-ured will be approximately correct. An alternative design might be to squeeze toothpaste directly onto pH paper strips.

Making Connections10. Examples of possible Did You Know?:

Some plants are acid-loving and others are not. For example, evergreen trees are acid-loving: they grow best in soilthat has a pH of less than 7 and will actually change the pH of the soil in which they grow. If you remove an ever-green tree from a lawn and plant grass in that area, the grass probably won’t grow: grass is not acid-loving. The soilwill have to be neutralized by adding a base, such as lime, to the soil.

Flowers on some plants will change colour depending on the pH of the soil. Examples are rhododendrons andazaleas, which are acid-loving but vary in colour depending upon the soil pH. The pH can be decreased into the acidicrange by adding citric fruit peels, pine needles, and peat moss to the soil.Some vegetables, such as cabbage, cauliflower, broccoli, and turnips, like to grow in alkaline soil with a pH of 7.5-8.0. Horticultural lime is used by gardeners to “sweeten” the sour soil.

You can test a sample of your soil with pH paper or a pH meter, or take the soil sample to a nearby greenhouse.11. An acid wash will clean the lettuce and also tend to remove pesticides and kill microorganisms on it. The likely acid

to use would be the one that is commonly edible: acetic acid. You can use a diluted vinegar wash in your sink or in aspray bottle.

Reflecting12. “Weak,” referring to acids, indicates the degree of ionization in water. It has nothing necessarily to do with concen-

tration.


8.3 WORKING WITH SOLUTIONS

PRACTICE

(Page 376)


1. Name Use

Erlenmeyer flask for temporarily containing reacting substances (specifically shaped to allow mixing of contents by swirling) and also for approximate measure of various volumes

graduated pipet for very precise addition of various (smaller) volumes

graduated cylinder for precise measurement of various (larger) volumes

volumetric flask and stopper for very precise measurement (and mixing) of a single specific volume

buret for very precise addition of various (small) volumes

graduated beaker for temporarily containing reacting substances, and for approximate measure of various (large) volumes

PRACTICE

(Page 377)

Making Connections2. Typical answers might include information such as:

Teaching Chemistry requires a minimum four years of university training, with five or six years usually seen as prefer-able. Specifically, such a person would require a B.Sc. degree in physical sciences, plus background in mathematics,as well as education course requirements. (Courses vary from province to province.) Teachers must be certified by theprovincial government. At publication date, a nationwide shortage of teachers is being felt in this subject area, makingfuture job prospects good.

GO TO www.science.nelson.com, Chemistry 11, Teacher Centre.8.4 ACID–BASE THEORIES

8.4 ACID – BASE THEORIES

PRACTICE

(Page 379)

Understanding Concepts1. Sour taste for acids is not an appropriate lab test, because many lab acids are dangerous: toxic and/or highly reactive.

It would be of practical use in a household kitchen.2. (a) Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g) fast (strong acid)

(b) Mg(s) + 2 HC2H3O2(aq) → Mg(C2H3O2)2(aq) + H2(g) slow (weak acid)

(c) 2 HCl(aq) + CaCO3(s) → H2O + CO2(g) + CaCl2(aq) fast (strong acid)

(d) 2 HC2H3O2(aq) + CaCO3(s) → Ca(C2H3O2)2(aq) + H2O(g) + CO2(g) slow (weak acid)

3. The best properties to distinguish strong acids from weak acids would be the rates of reaction with active metalsand/or carbonates, the last two properties listed in Table 1. The other tests would distinguish acids from bases, but notstrong acids from weak acids.

4. Strong acids are molecular substances that theoretically ionize in aqueous solution to a very large extent (essentiallycompletely), to produce hydrogen ions. Weak acids are molecular substances that theoretically ionize in aqueous solu-tion to a very small extent, to produce hydrogen ions.


5. (a) The concentration of hydrogen ions in the strong acid solution will be 100 ions per litre, and in the weak acidsolution will be 2 ions per litre.

(b) Strong acids react much faster and (in equal concentration) have a lower pH, because their solutions have morehydrogen ions present.


6. PredictionStrong bases should react much faster and (in equal concentration) have a higher pH than weak bases, becausetheir solutions should have more hydroxide ions present.

Experimental DesignSolutions of several bases (independent variable) of equal concentrations (controlled variable) will be tested forpH (dependent variable).

7. (a) Experimental DesignSolutions of several acids (independent variable) of equal concentrations (controlled variable) will be tested forpH (dependent variable).

(b) AnalysisAccording to pH values, in order of decreasing strength, the acids are:hydrochloric and nitric acid (equal in pH)hydrofluoric acidmethanoic acidethanoic acidhydrocyanic acid

8. Experimental DesignEqually concentrated solutions of the substances are tested for pH and for electrical conductivity.

Analysis of the DesignThe reasoning behind this design is that, of the two acidic solutions, HCl(aq) will have a lower pH than HC2H3O2(aq),because HCl(aq) is a strong acid, and HC2H3O2(aq) is a weak acid. Of the two neutral solutions, NaCl(aq) is ionic andwill conduct, and C12H22O11(aq) is molecular and will not conduct. The two bases can be distinguished by addingNa2SO4(aq) solution to each, which will precipitate BaSO4(s) from the Ba(OH)2(aq) solution, but not from the KOH(aq)solution.

Note: The Ba(OH)2(aq) solution will have a higher pH and conduct electricity better than the KOH(aq) solution,because the dissociation of Ba(OH)2(aq) produces two hydroxide ions per formula unit, and the dissociation of KOH(aq)produces only one hydroxide ion per formula unit (but the course of study in the text has not yet addressed this point).

Making Connections9. Personal experience indicates that acids “eat away” materials quite slowly — even those “instant” lime and scale

removers advertised on television. The stronger acids are more dangerous. Entertainment media are unlikely to portrayacid reactivity accurately; exaggeration is their “selling” point.

10. Acid deposition (acid rain) is acidic “fallout” from the atmosphere, initially mostly produced by vehicle exhausts andindustrial pollution sources. The two predominant acids are nitric and sulfuric (both strong acids). Nitrous andsulfurous acids (both weak acids) may also be present. All rainwater contains dissolved carbon dioxide (carbonicacid), so normal unpolluted rainwater has a pH of about 5.6 — very slightly acidic. You might predict that strong acidswould affect the environment more, but living systems are so complex that this would almost certainly be an over-simplification.

GO TO www.science.nelson.com, Chemistry 11, Teacher Centre.

Reflecting11. The maximum concentration of the hydrogen ions in a 0.01 mol/L acid solution is (with a few exceptions) 0.01 mol/L.

This maximum concentration will be attained by strong acids but not by weak acids. There is no way to predict whatthe H+

(aq) concentration will be in a 0.01 mol/L unknown acid solution, with the concepts studied to this point. If theacid were identified as strong, or the % ionization value were provided, that information would aid the accuracy ofprediction.


PRACTICE

(Page 386)

Understanding Concepts12. Early ideas about acids were that they were hydrogen compounds that ionized in water to produce hydrogen ions.

However, some acidic solutions were found to be solutions of compounds that contain no hydrogen atoms.Furthermore, hydrogen ions are theoretically impossible structures.

13. In the original Arrhenius theory, acids are molecular substances that ionize in aqueous solution to produce hydrogenions, and bases are ionic substances that dissociate in aqueous solution to produce hydroxide ions. This theory predictsand explains quite a few acidic and basic substances, but there are lots of exceptions — too many to leave the theoryunrevised.

14. We now assume acidic solution properties are due to the presence of hydrated protons, commonly called hydroniumions, symbolized H3O+

(aq).15. (a) HCN(aq) � H2O(l) → H3O+

(aq) � CN–(aq)

(b) HNO3(aq) � H2O(l) → H3O+(aq) � NO–

3(aq)

(c) Na2SO4(aq) → 2 Na+(aq) � SO2–

4(aq) followed by

SO2–4(aq) � H2O(l) → HSO–

4(aq) � OH–(aq)

(d) Sr(OH)2(aq) → Sr2+(aq) � 2 OH–

(aq)

16. (a) weak acid(b) strong acid(c) weak base(d) strong base

PRACTICE

(Page 389)

Understanding Concepts17. According to Brønsted-Lowry definitions, acids differ from bases in that acids lose (donate) protons in a proton-

transfer reaction, and bases gain (accept) protons in a proton-transfer reaction.18. (Reactants are shown in bold type.)

(a) The acids are: HF(aq) and HSO–3(aq)

The bases are: F–(aq) and SO2–

3(aq)

(b) The acids are: HC2H3O2(aq) and HCO–3(aq)

The bases are: C2H3O–2(aq) and CO2–

3(aq)

(c) The acids are: H3PO4(aq) and HOCl(aq)

The bases are: H2PO–4(aq) and OCl–(aq)

19. (a) HSO–4(aq) � HCO–

3(aq) → SO2–4(aq) � H2CO3(aq)

(b) One acid–base pair is: HSO–4(aq) and SO2–

4(aq)

and the other pair is: H2CO3(aq) and HCO–3(aq)

20. Brønsted-Lowry theory removes restrictions to acid–base reactions in that the theory does not depend on thecharacteristics of solutions, nor does it require that water be a solvent, or, in fact, that the reactants be dissolved.


SECTIONS 8.3–8.4 QUESTIONS

(Page 392)

Understanding Concepts1. In strong acids a high percentage of molecules react with water to form hydronium ions. In weak acids a low

percentage of molecules react with water to form hydronium ions. It is this difference in hydronium ion concentrationthat gives strong and weak acids their distinctive properties.

2. Soluble ionic hydroxides are strong bases, as indicated by their empirical properties in solution — they not only tastebitter, feel slippery, turn red litmus blue, and neutralize acids like all bases, but they also have very high pH values(eleven or above) and electrical conductivity, and react rapidly (compared to weak bases with equal concentration).The latter high pH, conductivity, and reaction rate for strong bases are explained by theorizing that they exist (100%)in water as hydroxide ions and electrically balancing cations (e.g., hydroxide ions and sodium ions).

3. Weak bases have the properties of strong bases, but to a lesser degree. In particular, compared to strong base solutionswith equal concentration, weak bases form aqueous solutions with pH values above seven (but not above, say, ten),and have conductivity and reaction rates that are low. This is explained by the theory that weak bases are substancesthat, when they dissolve, react only partially with water to form a low percentage of hydroxide ions in solution.

4. (a) HBr(g) � H2O(l) → H3O+(aq) � Br–

(aq)

(b) KOH(s) → K+(aq) � OH–

(aq)

(c) HC7H5O2(s) � H2O(l) → H3O+(aq) � C7H5O–

2(aq)

(d) Na2S(s) → 2 Na+(aq) � S2–

(aq) followed by S2–(aq) � H2O(l) → HS–

(aq) � OH–(aq)

5. (a) According to the Arrhenius concept, acids are substances that dissolve in water to produce hydrogen ions.(b) According to the revised Arrhenius concept, acids are substances that react with water to produce hydronium

ions.(c) According to the Brønsted-Lowry concept, acids are substances that donate protons in proton-transfer reactions.

6. (a) According to the Arrhenius concept, bases are substances that dissolve in water to produce hydroxide ions. Weakbases are not explained by this concept.

(b) According to the revised Arrhenius concept, bases are substances that dissolve in water or react with water to produce hydroxide ions. Weak bases react only slightly with water.

(c) According to the Brønsted-Lowry concept, bases are substances that accept protons in proton-transfer reactions.Weak bases attract protons less strongly than strong bases.

7. According to the Brønsted-Lowry concept, an acid–base reaction involves the transfer of a proton from the acid to thebase.

8. (a) Both of the acids and bases in the reaction are listed. The reactant acid and base asked for in the question are givenin bold type.The acids are HCO–

3(aq) and HS–(aq)

The bases are S2–(aq) and CO2–

3(aq)

(b) The acids are H2CO3(aq) and H2O(l)

The bases are OH–(aq) and HCO–

3(aq)

9. (a)HSO–4(aq) � PO3–

4(aq) → SO2–4(aq) � HPO2–

4(aq)

(b)H3O+(aq) � HPO2–

4(aq) → H2O(aq) � H2PO–4(aq)

10. One acid–base pair is HCO–3(aq) and CO2–

3(aq),

and the other pair is H2CO3(aq) and HCO–3(aq)

11. (a) One acid–base pair is H3O+(aq) and H2O(aq),

and the other pair is H2SO3(aq) and HSO–3(aq)

(b) One acid–base pair is H2O(aq) and OH–(aq),

and the other pair is HSO–3(aq) and SO2–

3(aq)


Applying Inquiry Skills12. Baking soda will react both with strong bases and with acids. It releases carbon dioxide gas upon reaction with acids.

Since it can react either as an acid or as a base, it is not simple to predict how it will react in any given situation.


Making Connections13. Uses of baking soda include:

baking — reacts with food acids to produce CO2(g) for leaveningbrushing teeth — a mild non-abrasive non-toxic cleaneracid spills — neutralizes them for cleanupbase spills — neutralizes them for cleanupodour removal — reacts with acidic or basic odorous gases in refrigerators, kitchens, and carpetsfirefighting — releases carbon dioxide, which smothers flamescleaning — makes a solution for washing surfaces

8.5 ACID–BASE REACTIONS

PRACTICE

(Page 394)

Understanding Concepts1. Acids react with active metals to produce hydrogen and an ionic compound; react with carbonate compounds to

produce carbon dioxide gas and water; and neutralize bases to produce water.2. (a) 2 HBr(aq) � Zn(s) → H2(g) � ZnBr2(aq)

HBr(aq) � NaOH(s) → H2O(l) � NaBr(aq)

2 HBr(aq) � Na2CO3(s) → H2CO3(aq) � 2 NaBr(aq)

or 2 HBr(aq) � Na2CO3(s) → H2O(l) � CO2(g) � 2 NaBr(aq)

(b) The first neutralization produces hydrogen, which is flammable and dangerous, and uses zinc, which is notcommonly available.The second neutralization uses lye (a strong base), which is very corrosive and not easy to handle.The third neutralization is practical. It uses washing soda, which is non-hazardous, inexpensive, and commonlyavailable; and produces no dangerous products.

3. (a) 3 H2C2O4(aq) � 2 Al(s) → 3 H2(g) � Al2(C2O4)3(s)

(b) H2C2O4(aq) � CaCl2(aq) → 2 HCl(aq) � CaC2O4(s)

or H2C2O4(aq) � Ca2+(aq) → 2 H+

(aq) � CaC2O4(s)

(c) 3 H2C2O4(aq) � FeCl3(aq) → 6 HCl(aq) � Fe2(C2O4)3(s)

or 3 H2C2O4(aq) � Fe3+(aq) → 6 H+

(aq) � Fe2(C2O4)3(s)

Iron(III) ions are removed from solution by reaction with oxalate, effectively preventing the body from using them.

Practice

(Page 399)

Understanding Concepts4. Acids react with active metals to produce hydrogen and an ionic compound; react with carbonate compounds to

produce carbon dioxide gas and water; and neutralize bases to produce water.5. The requirements are that the chemical reaction must be spontaneous, rapid, quantitative, and stoichiometric.6. The two reactants in a titration are the sample, usually in an Erlenmeyer flask, and the titrant, usually in a buret.7. A standard solution is one with a precisely known concentration.


8. Repetition in titration — as elsewhere in science — increases the reliability of the answer. Mistakes can be easily iden-tified and omitted, and averaging several measured values always reduces the effect of normal experimental error.

Applying Inquiry Skills9. 2 KOH(aq) � H2SO4(aq) → K2SO4(aq) � 2 H20(l)

9.44 mL 10.00 mL

0.0506 mol/L C

nKOH � 9.44 mL� � �0.05

106

L�mol

� � 0.478 mmol

nH2SO4� 0.478 mmol � �

12

� � 0.239 mmol

CH2SO4� �

01.203.090m�m�mLol

�

CH2SO4� 0.0239 mol/L = 23.9 mmol/L

or CH2SO4� 9.44 mL� KOH� ��

0.05106

LmK

oOl�H�KOH�

�� 1

2

m

m

o

o

l

l�H

K2

O

SO

H�4� � �

10.010 mL��

CH2SO4� 0.0239 mol/L = 23.9 mmol/L

The concentration of sulfuric acid in the water is 23.9 mmol/L.

10. (a) Evidence

Table 4: Volume of 0.0161 mol/L Sodium Hydroxide Required to Neutralize 10.00 mL of Diluted Oxalic Acid

Trial 1 2 3 4

Final buret reading (mL) 14.3 27.8 41.1 13.8

Initial buret reading (mL) 0.2 14.3 27.8 0.4

Volume of NaOH(aq) used (mL) 14.1 13.5 13.3 13.4

(b) Analysis2 NaOH(aq) � H2C2O4(aq) → K2C2O4(aq) � 2 H20(l)

13.4 mL 10.00 mL

0.0161 mol/L C

nNaOH � 13.4 mL� � �0.01

161

L�mol

� � 0.216 mmol

nH2C2O4� 0.216 mmol � �

12

� � 0.108 mmol

CH2C2O4� �

01.100.080m�m�mLol

�

CH2C2O4� 0.0108 mol/L

or CH2C2O4� 13.4 mL� NaOH� � �

0.01161

LmN

oal�ONH�aOH�

��1

2

m

m

o

o

l

l�H

N2C

aO2O

H�4��

10.010 mL��

CH2C2O4� 0.0108 mol/L

The concentration of oxalic acid in the rust remover is 100 times the concentration of the diluted acid used in thetitration, or 1.08 mol/L.

(c) Evaluationdifference � experimental value � predicted value

� 1.08 mol/L � 1.11 mol/L

difference � � 0.03 mol/L

% difference = �pre

ddiifcfeterden

vcaelue

� � 100%


= �01..0131 m

mooll//L�L�

� � 100%

= 3%

The % difference is 3%, which is quite acceptable for school lab work.The prediction based on the manufacturer’s label is quite accurate from a scientific point of view: the

percentage difference is only 3%, which is quite acceptable.Note: You might discuss with students that from a legal perspective there could be a problem. A commercial

label is a legally guaranteed minimum unless specifically stated otherwise, so if the solution really has a concentration below 1.11 mol/L, the manufacturer of the rust remover may be in trouble.

11. (a) Evidence

Table 5: Titration of 10.00-mL Samples of HCl(aq)with 0.974 mol/L Ba(OH)2(aq)

Trial 1 2 3 4



Volume of Ba(OH)2(aq) added (mL) 15.0 13.7 13.7 13.6

Colour at endpoint blue green green green

(b) Analysis

Ba(OH)2(aq) � 2 HCl(aq) → BaCl2(aq) � 2 H20(l)

13.7 mL 10.00 mL

0.974 mol/L C

nBa(OH)2� 13.7 mL� � �

0.9714L�mol� � 13.3 mmol

nHCl � 13.3 mmol � �21

� � 26.6 mmol

CHCl � �2160..600

m�m�m

Lol

�

CHCl � 2.66 mol/L

or CHCl � 13.7 mL� Ba(OH)2� � ��1 m

2

o

m

l�o

B

l

a

H

(O

C

H�l

)2��

10.010 mL��

CHCl � 2.66 mol/L

The concentration of hydrochloric acid is 2.66 mol/L.12. 2 NaOH(aq) � H2SO4(aq) → Na2SO4(aq) � 2 H20(l)

11.48 mL 10.00 mL

0.484 mol/L C

nNaOH � 11.48 mL� � �0.48

14L�mol� � 5.56 mmol

nH2SO4� 5.56 mmol � �

12

� � 2.78 mmol

CH2SO4� �

21.07.800

m�m�m

Lol

�

CH2SO4� 0.278 mol/L

or CH2SO4 �11.48 mL� NaOH� ��

0.4814Lm

Noal�ON

H�aOH�

�� 1

2

m

m

o

o

l

l�H

N2

a

S

O

O

H�4� � �

10.0

1

0 mL��

CH2SO4� 0.278 mol/L

The concentration of sulfuric acid is 0.278 mol/L.

0.974 mol� Ba(OH�)2��


SECTION 8.5 QUESTIONS

(Page 401)

Understanding Concepts1. Answers will vary, but may include the following typical acid reactions.

Reaction with active metals:2 HCl(aq) � Mg(s) → H2(g) � MgCl2(aq)

Reaction with strong bases:

HNO3(aq) � NaOH(s) → H2O(l) � NaNO3(aq)

Reaction with carbonate compounds:

2 HCl(aq) � K2CO3(s) → H2CO3(aq) � 2 KCl(aq)

or 2 HCl(aq) � K2CO3(s) → H2O(l) � CO2(g) � 2 KCl(aq)

2. Al(OH)3(s) � 3 HCl(aq) → AlCl3(aq) � 3 H2O(l)

0.912 g v

78.01 g/mol 0.10 mol/L

nAl(OH)3� 0.912 g� � �

718.

m01

olg�

� � 0.0117 mol

nHCl � 0.0117 mol � �31

� � 0.0351 mol

vHCl � 0.0351 mol� � �0.1

10

Lmol��

vHCl � 0.35 L

or vHCl � 0.912 gg� Al(OH)3� � �7

1

8.

m

01

ol�g�A

A

l(

l

O

(O

H�H�)3

)3� � �

1 m

3

o

m

l�o

A

l�l

H

(O

C

H�l�)3

� ��0.1

10

Lm

Hol�

CHl

Cl��

vHCl � 0.35 L

The volume of hydrochloric acid neutralized is 0.35L.3. Ca(OH)2(s) � H2SO4(aq) → CaSO4(s) � 2 H2O(l)

1.0 � 106 g v

74.10 g/mol 1.2 �10–3 mol/L

nCa(OH)2� 1.0 � 106 g� � �

714.

m10

olg�

� � 1.4 �104 mol

nH2SO4� 1.4 �104 mol � �

11

� � 1.4 �104 mol

vH2SO4� 1.4 �104 mol� ��

1.2 �

110

L–3 mol��

vH2SO4� 1.1 �107 L

or vH2SO4� 1.0 � 106 g� Ca(OH)2� ��

714.

m10

ol�g�

CCaa((OOHH))2�

2��

1

1

m

m

ol�o

C

l� H

a(2

O

S

H�O

)4�

2� �

vH2SO4� 1.1 �107 L

The volume of lake water sulfuric acid that can be neutralized is 1.1 � 107 L (or 1.1 � 104 m3).

1 L H2SO4��



4. Experimental Design (1)A sample of sodium hydroxide solution is titrated with standard hydrochloric acid, and the concentration of sodiumhydroxide is calculated from the measured volume of hydrochloric acid using the stoichiometric method.

Materials (1)• lab apron

• eye protection

• standard HCl(aq)

• NaOH(aq)

• bromothymol blue


• 100-mL or 150-mL beake

• 250-mL beaker

• 50-mL buret

• 10-mL volumetric pipet

• pipet bulb

• ring stand

• buret clamp

• stirring rod

• small funnel

• 250-mL Erlenmeyer flask

• meniscus finder

Experimental Design (2)A sample of sodium hydroxide solution is reacted with excess lead(II) nitrate solution, and the precipitate is filteredand dried; and the concentration of sodium hydroxide is calculated from the measured mass of lead(II) hydroxideprecipitate using the stoichiometric method.

Materials (2)• lab apron

• eye protection

• standard Pb(NO3)2(aq)

• NaOH(aq)


• 250-mL beaker

• 400-mL beaker


• pipet bulb

• ring stand

• iron ring

• stirring rod

• small funnel

• filter paper

• centigram balance


5. Experimental Design

A sample of oxalic acid solution is titrated with standard sodium hydroxide solution, and the concentration of oxalicacid is calculated from the measured volume of sodium hydroxide using the stoichiometric method.

Materials (1)

• lab apron

• eye protection

• standard NaOH(aq) (known concentration)

• H2C2O4(aq)

• bromothymol blue


• 100-mL beaker

• 150-mL beaker

• 50-mL buret


• pipet bulb

• ring stand

• buret clamp

• tirring rod

• small funnel

• 250-mL Erlenmeyer flask

• meniscus finder

• toxic waste disposal container

Procedure1. Obtain about 50 mL of oxalic acid in a clean, dry 100-mL beaker.2. Obtain about 70 mL of NaOH(aq) in a clean, dry, labelled 150-mL beaker.3. Set up the buret with NaOH(aq) following the accepted procedure for rinsing and clearing the air bubble

(see Skills Handbook).4. Pipet a 10.00-mL sample of oxalic acid into a clean Erlenmeyer flask.

(Caution: Oxalic acid is toxic. Use a pipet bulb.)5. Add 1 or 2 drops of bromothymol blue indicator.6. Record the initial buret reading to the nearest 0.1 mL.7. Titrate the sample with NaOH(aq) until a single drop produces a permanent change from pale yellow to pale blue.8. Record the final buret reading to the nearest 0.1 mL.9. Dispose of the flask contents by rinsing into a toxic waste container.10. Repeat steps 4 to 9 until three consistent results (to � 0.1 mL) are obtained.


CHAPTER 8 SUMMARY

(Page 402)

Make a Summary

Reflect on your LearningHere is an example:My thinking on what an acid and a base are has changed from an Arrhenius (ionization) concept to a Bro�nsted-Lowry(proton-transfer) concept. I still see value in the Arrhenius concept and will continue to use it whenever it is suitable.

I now understand the difference between a strong and weak acid and a strong and weak base. Now I have to rememberto distinguish between strength and concentration. This takes mental effort

The fact that sometimes a chemical can act as an acid in one reaction and as a base in another reaction is interesting.I now see how important empirical evidence is to the study of chemistry. The ultimate way of knowing in chemistry isempirical, not theoretical as I had previously thought. Now I see where chemical knowledge really comes from. This iswhat is exciting to me.

Titrations are neat. I especially like the sudden colour change at the endpoint, although I often overshoot the endpoint.What I don’t understand is how to choose the right indicator from that long list at the end of the book, and how do indi-cators work anyway? Why do they change colour?

hydrochloric acid

buret

stop cock

Erlenmeyer flask

• titrant

• standard solution

• known concentration (0.12 mol/L)

• strong acid (high conductivity;

� fast reaction)

• 100% ionized (Arrhenius theory)

• > 99% reacted with water

� (Brønsted-Lowry theory)

• very low pH

gripewater •�unknown concentration

•�NaHCO3(s) dissociates

•�HCO3(aq) is amphiprotic with

� conjugate acid-base pairs of

� HCO3(aq) – H2CO3(aq) (here) and

� HCO3(aq) – CO3(aq)

or

or

HCL(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)

H3O(aq) + Cl(aq) + Na(aq) + HCO3(aq) →

H2CO3(aq) + Na(aq) + Cl(aq) (total ionic)

H3O(aq) + HCO3(s) → H2O(l) + H2CO3(aq)

H+

• pH = –log [H(aq)] = – log 0.12 = 0.92+

• HCl(aq) → H2O(1) + H3O(aq) + Cl(aq)–+

• HCl(aq)→ H(aq) + Cl(aq)–+100%

99%

•�HCO3(aq) is a (weak) base (here)–

–

–

– 2–

•�NaHCO3(s) → Na(aq) + HCO3(aq)–+

–

+ –

–+

+

proton transfer

indicator •�endpoint provides empirical

� evidence for the end of

� the acid-base reaction

conjugate pair

conjugate pair

+ –

acid base base acid


CHAPTER 8 REVIEW

(Page 403)

Understanding Concepts1. (a) An acid is a substance that dissolves to form a conducting solution that turns blue litmus red, neutralizes bases,

reacts with active metals to form hydrogen gas, and reacts with carbonate compounds to form carbon dioxide gas.(b) A base is a substance that dissolves to form a conducting solution that turns red litmus blue, and neutralizes acids.

2 (a) sodium hydroxide base(b) acetic acid acid(c) magnesium hydroxide base(d) hydrochloric acid acid(e) calcium hydroxide base(f) (aqueous) ammonia base\

3 (a) basic: NaOH(s) → Na+(aq) � OH–

(aq)

(b) acidic: HC2H3O2(aq) → H+(aq) � C2H3O–

2(aq)

(c) basic: Mg(OH)2(s) → Mg2+(aq) � 2 OH–

(aq)

(d) acidic: HCl(aq) → H+(aq) � Cl–(aq)

(e) basic: Ca(OH)2(s) → Ca2+(aq) � 2 OH–

(aq)

(f) basic: NH3(aq) � H2O(l) → NH+4(aq) � OH–

(aq)

4. (a) [H+(aq)] is much greater in the hydrochloric acid solution.

(b) In hydrochloric acid, essentially all of the HCl molecules are dissociated into ions. In acetic acid, only a verysmall percentage of HC2H3O2 molecules are dissociated.

(c) The same volume of NaOH(aq) would be required for each neutralization reaction.5. In order of increasing pH, the solutions are:

HCl(aq), HC2H3O2(aq), NaCl(aq), NH3(aq), NaOH(aq)

6. (a) pH � –log [H+(aq)]

� –log [7.5 � 10–3 mol/L]

pH � 2.12

(b) pH � –log [H+(aq)]

� –log [2.5 � 10–3 mol/L]

pH � 2.60

7. (a) [H+(aq)] � 10–pH

� 10–11.56 mol/L

[H+(aq)] � 2.8 3 10–12 mol/L

(b) [H+(aq)] � 10–pH

� 10–3.50 mol/L

[H+(aq)] � 3.2 � 10–4 mol/L

8. Electrical conductivity, pH, and rate of reaction with active metals can all be used to rank acids in terms ofstrength.

9. (a) HNO2(aq) has a higher pH (is less acidic, more basic).

<50%

(b) HNO2(aq) � H2O(l) → H3O+(aq) � NO–

2(aq)

>99%


HNO3(aq) � H2O(l) → H3O+(aq) � NO–

3(aq)

As shown, nitric acid transfers protons to water completely, whereas in nitrous acid the transfer is much less than50%, making the solution much less acidic.

10. [H+(aq)] is ten times as concentrated in pH 5 as in pH 6.

11. Concept Main Idea Limitationsoxygen acids contain oxygen and doesn’t explain solutions of HCl(g), or why some oxides make

react with limestone basic solutions

hydrogen acids contain hydrogen and doesn’t explain acidic solutions of SO2(g), or basic solutions from react with active metals to compounds containing hydrogen, e.g., NH3(g)produce hydrogen gas

Arrhenius acids dissolve in water to doesn’t explain nonaqueous solutions, or hydrogen produce H+(aq) containing substances that can neutralize both acids and bases

12. acid base conjugate acid conjugate base

(a) HSO–4(aq) HCO–

3(aq) →H2CO3(aq) SO2–4(aq)

(b) HSO–4(aq) HPO2–

4(aq) →H2PO–3(aq) SO2–

4(aq)

(c) H2PO–4(aq) H2BO–

3(aq) →H3BO3(aq) HPO2–4(aq)

(d) HCO–3(aq) HS–

(aq) →H2S(aq) CO2–3(aq)

(e) HSO–3(aq) NH3(aq) →NH+

4(aq) SO2–3(aq)

13. (a) NH3(aq) ammonia B-L acid or base

(b) NH+4(aq) ammonium ion B-L acid

(c) NO–2(aq) nitrite ion B-L base

(d) NO–3(aq) nitrate ion B-L base

14. (a) conjugate acid–base pairs are: PO3�4(aq) and HPO2�

4(aq)

and: H2O(aq) and OH�(aq)

(b) PO3–4(aq) is a strong base that can only react as a base with water, so it must form a basic solution.

15. (a) H2SO3(aq) + OH�(aq) → HSO–

3(aq) + H2O(l)

(b) HSO�3(aq) + OH�

(aq) → SO2–3(aq) + H2O(l)

16. (a) NaHCO3(aq) + HCl(aq) → H2CO3(aq) + NaCl(aq)

HCO–3(aq) + H3O+

(aq) → H2CO3(aq) + H2O(l)

(b) NaHCO3(aq) + NaOH(aq) → Na2CO3(aq) + H2O(l)

HCO–3(aq) + OH�

(aq) → CO2–3(aq) + H2O(l)

(c) HCO�3(aq) + H3O+

(aq) → H2CO3(aq) + H2O(l)

B A A B

HCO–3(aq) + OH�

(aq) → CO2–3(aq) + H2O(l)

A B B A

The conjugate acid–base pairs are:H3O+

(aq) and H2O(l) and H2CO3(aq) and HCO�3(aq)

H2O(l) and OH�(aq) and HCO–

3(aq) and CO2–3(aq)

(d) The hydrogen carbonate ion may act as either an acid or as a base; it is amphiprotic.17. A chemical reaction suitable for titration must be spontaneous, fast, stoichiometric, and quantitative.18. You must know the concentration of one reactant solution accurately to calculate precise results.


19. (a) titration: a laboratory procedure involving the carefully measured and controlled addition of a standard solutionfrom a buret into a measured volume of a sample solution (or the sample solution could be in the buret ...)

(b) titrant: the solution in the buret during a titration(c) endpoint: the point in a titration at which a sharp change in a property occurs (e.g., a colour change)

20. The acid reacts with an active metal to produce hydrogen gas.H2SO4(aq) + Zn(s) → ZnSO4(aq) + H2(g)

The acid is partially neutralized by a basic solution.H2SO4(aq) + Na2CO3(aq) → Na2SO4(aq) + H2CO3(aq)

The acid is completely neutralized by a strong basic solution.H2SO4(aq) + Zn(s) → ZnSO4(aq) + H2(g)

Applying Inquiry Skills21. The equipment required for the precision given would be:

(a) a volumetric flask(b) a 10-mL graduated cylinder or a 10-mL graduated pipet(c) a 10-mL volumetric pipet

22. Experimental DesignThe pH is measured for samples of an acid and of a base. The samples are diluted tenfold, and the pH measured again.

Materials• HCl(aq)

• NaOH(aq)

• two 150-mL beakers

• 100-mL graduated cylinder

• pH meter

23. (a) AnalysisSolution 1 is NaHCO3(aq), because it is basic, but not with the highest pH, and conducts well.Solution 2 is KNO3(aq), because it is neutral, and conducts well.Solution 3 is H2SO3(aq), because it is acidic, but not with the lowest pH, and conducts poorly.Solution 4 is HCl(aq), because it is acidic, with the lowest pH, and conducts well.Solution 5 is NaOH(aq) , because it is basic, with the highest pH, and conducts well.

(b) EvaluationThe experimental design is judged adequate because the experiment produces evidence needed to answer thequestion with a high degree of certainty.

24. (a) EvidenceTable 2: Titration of 10.00 mL of 0.120 mol/L Na2CO3(aq) with HCl(aq)

Trial 1 2 3 4



Volume of HCl(aq) added 17.6 17.1 17.0 17.2

Colour at endpoint red orange orange orange

(b) AnalysisNa2CO3(aq) � 2 HCl(aq) → H2CO3(aq) � 2 NaCl(aq)

10.00 mL 17.1 mL

0.120 mol/L C

nNa2CO3� 10.00 mL� � �

0.1210L�mol� � 1.20 mmol

nHCl � 1.20 mmol � �21

� � 2.40 mmol


CHCl � �21.470.1

m�m�mLol

�

CHCl � 0.140 mol/L

or CHCl � 10.00 mL� Na2CO3� ��0.12

1

0

L

m

N

o

a

l�

2

N

C

a

O�2

3

CO�3� ��1 m

2 m

ol�o

N

l

a

H

2C

C

O�l

3

� � �17.1

1mL��

CHCl � 0.140 mol/L

The concentration of the hydrochloric acid solution is 0.140 mol/L.

25. (a) Evidence

Table 3: Titration of 10.00 mL of H3PO4(aq) with 1.25 mol/L NaOH(aq)

Trial 1 2 3 4



Volume of NaOH(aq) added 12.9 11.7 11.8 11.6

Colour at endpoint deep red pale pink pale pink pale pink

(b) Analysis

H3PO4(aq) � 2 NaOH(aq) → H2CO3(aq) � 2 NaCl(aq)

10.00 mL 11.7 mL

C 1.25 mol/L

nNaOH � 11.7 mL� � �1.2

15

L�mol� � 14.6 mmol

nH3PO4� 14.6 mmol � �

12

�

nH3PO4� 7.31 mmol

CH3PO4� �

71.03.100

m�m�m

Lol

�

CH3PO4� 0.731 mol/L

or CH3PO4 � 11.7 mL� NaOH� � �

1.215Lm

Nol�

aO�N

HaOH�

� � �1

2

m

m

o

o

l

l�H

N3

a

P

O

O

H�4� � �

10.010 mL��

CH3PO4� 0.731 mol/L

The concentration of the original phosphoric acid solution in the rust remover is 7.31 mol/L — ten times theconcentration of the diluted titrated solution.

(c) EvaluationThe prediction, based on the manufacturer’s label, is verified by the results. The label seems to be quite accurate.

Making Connections26. Personal experience indicates that acids “eat away” materials quite slowly — even the “instant” lime and scale

cleaners advertised on television. The student response to this question will normally indicate that the movieindustry generally greatly exaggerates (or even completely fabricates) the reactivity of acids for effect. Moviesfeaturing entertaining misconceptions about acids include:

The Plastic Man Comedy/Adventure Show

Contaminazione

The Navy vs. the Night Monsters

Alien, Aliens27. Boric acid is a very weak acid, so that almost none of its molecules in solution react with water to produce hydro-

nium ions.


<50%

H3BO3(aq) � H2O(l) → H3O+(aq) � H2BO3

–(aq)

For strong acids like HCl(aq), the dissolved molecules react with water almost totally, producing a high concentra-tion of reactive hydronium ions.

>99%

HCl(aq) � H2O(l) → H3O+(aq) � Cl–(aq)

28. Experimental DesignThe scale is removed by reaction with hydrochloric (shown here) or any other acid. A weaker acid reacts the sameway, but more slowly — and would be much safer to work with. The reasoning behind this design is presented below.The hard-water scale (CaCO3(s) and/or MgCO3(s)) reacts with the acid to form soluble compounds, as shown in thefollowing reaction equations.CaCO3(s) � 2 HCl(aq) → CaCl2(aq) � H2CO3(aq)

MgCO3(s) � 2 HCl(aq) → MgCl2(aq) � H2CO3(aq)

Materials• eye protection

• rubber gloves

• hydrochloric acid (commercial scale remover or 0.10 mol/L)

• tap water

• plastic or glass bowls/containers

• plastic sponge

Procedure1. Wear eye protection and rubber gloves.2. Soak smaller scale-coated parts in the acid in a glass or plastic container.3. Pour the acid solution into containers such as kettles that need to be descaled.4. Wash larger scale-coated parts with the acid using a plastic sponge, being careful not to allow dripping onto other

surfaces.5. Rinse all solutions thoroughly down the sink with plenty of water.

29. Students will normally find that pH adjustment of municipal water is straightforward: any cheap non-toxic acid orbase will do. Sodium hydroxide will increase pH while hydrochloric acid will decrease it, without adding any harmfulions in the process (providing only very low concentrations are achieved).


30. Conifers have evolved to use nutrients that are most readily present in soils with lower pH values. Acid rain in generallowers the pH in plants more than the normal level, resulting in varying degrees of stress for the plants.


Exploring31. Lewis acid–base theory focuses on electron pairs in bond formation, with an acid being the electron pair acceptor, and

a base the electron pair donor. This concept allows dealing with many reactions that do not involve Brønsted-Lowryacids or bases.



8.1 understanding acids and bases - quia

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