date : acid/base reactions theory booklet 3€¦ · in an acid-base titration (tick represents the...

Date :

Acid/Base Reactions

Theory Booklet 3 IQ3: Quantitative Analysis

Name : ______________

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Copyright 2019 SKY HSC College 1 All rights reserved

SYLLABUS COVERED ............................................................................................................................................. 2

1.1 QUANTITATIVE ANALYSIS ................................................................................................................................. 2

1.2 ACID-BASE TITRATION .................................................................................................................................... 6

1.3 PRIMARY STANDARD & STANDARD SOLUTION ..................................................................................................... 8

1.4 EQUIVALENCE POINT VS END POINT .................................................................................................................. 9

1.5 PRAC: PREPARING STANDARD SOLUTIONS ........................................................................................................ 10

1.6 APPROPRIATE APPARATUS FOR TITRATION ........................................................................................................ 14

1.7 JUSTIFYING THE METHOD FOR TITRATION ......................................................................................................... 16

1.8 PRAC: PERFORMING A TITRATION (STANDARDISING) .......................................................................................... 18

Homework Exercise 1 ........................................................................................................................ 22

Exam Style Questions 1 ..................................................................................................................... 35

1.9 TITRATION CURVES & SUITABLE INDICATORS ..................................................................................................... 37



1.10 TITRATIONS & ELECTRICAL CONDUCTIVITY ...................................................................................................... 54

1.11 BACK TITRATIONS ...................................................................................................................................... 58

1.12 ERRORS IN TITRATIONS ............................................................................................................................... 63



SYLLABUS COVERED ........................................................................................................................................... 72

2.1 MODELLING BY USING AN EXPERIMENT ............................................................................................................ 72

2.2 MODELLING BY USING MOLECULAR MODEL KITS ............................................................................................... 74

2.3 MODELLING USING AN ANIMATION ................................................................................................................ 75

Sample


What is Acid-Base Titration?

is a method of volumetric analysis which uses neutralisation reaction between a solution

of known volume and known concentration and a solution of known volume but unknown concentrations as a

means to accurately determine the unknown concentration of the latter solution.

The table on the right summarises the information involved

in an acid-base titration (tick represents the known values).

The calculations involved to determine the concentration of

solution 2 is of identical nature to the revision questions

done earlier (i.e. students have already done them).

However, the study of titrations in this module goes beyond just simple calculations. It is heavily procedure centric

(i.e. students should focus on understanding the appropriate procedures of a titration including selecting suitable

apparatus and the reasons why a given methodology is appropriate while another is not).

: Acid-base titrations are not the only form of volumetric analysis, nor are they the only form of titrations.

When studying this module, students will often see the term titration used to mean an acid-base titration. This is

because students study titrations under the “Acid/Base Reactions” module. However, students must be aware that

there are also other forms of titrations (e.g. redox and precipitation titrations).

Overview

1. An accurately known volume of either an acid or a base of interest is placed in a conical flask using a pipette.

2. A few drops of an appropriate indicator are placed in the flask so that a colour change will occur when the neutralisation reaction between the acid and base is complete.

3. The other solution is then added to the flask from a burette, which is a long tube with calibrated volume markings on the side and a tap at the bottom.

4. Sufficient solution is added from the burette to completely react with the solution in the flask.

5. From the measured volume and known concentration of the solution used, it is possible to calculate the concentration of the solution being analysed.

A Balanced Equation 𝒙 𝐀𝐜𝐢𝐝 𝒚 𝐁𝐚𝐬𝐞 → 𝒂 𝐒𝐚𝐥𝐭 + 𝒃 𝐖𝐚𝐭𝐞𝐫

Stoichiometric (mole) Ratio 𝒙 𝒚 𝒂 : 𝒃

𝒄 (to be determined)

(as it is a standard solution) It doesn’t matter which solution is placed in the pipette or burette as long as the concentration of one solution is given since volumes of the both solutions will become known eventually (either from pipette or a difference in readings of the burette) through the experiment.

𝑽 usually 25 mL = 0.025 L from the pipette

(will be measured from

the readings on the burette)

𝒏

using the stoichiometric ratio between acid and base

n(acid) = n(base) ×𝒙

𝒚

from

𝑛(𝑏𝑎𝑠𝑒)= 𝑐(𝑏𝑎𝑠𝑒) 𝑉(𝑏𝑎𝑠𝑒)

Concentration Volume

Solution 1 ✓ ✓

Solution 2 ? ✓

1 2

3

Sample


Measure the volume of titrant (i.e. titre, see below).

① Determine the number of moles of titrant added to the conical flask (i.e. no. of moles reacted with the analyte).

② Use stoichiometric ratio to determine 𝒏(𝒂𝒏𝒂𝒍𝒚𝒕𝒆).

③ Use the formula c =n

V to determine [𝐚𝐧𝐚𝐥𝐲𝐭𝐞].

Important Terminology

Terms Meaning

Titrant the solution in the burette that is added to the analyte. It is usually the solution of accurately known concentration (i.e. a standard solution).

Titre the volume of titrant used in the titration

Analyte

It is usually the solution whose concentration is unknown and hence to be determined in the analysis (Or simply the solution in the conical flask to which the titrant is added)

Aliquot volume of analyte used in the analysis

Acid-base titration is a type of quantitative analysis which is the analysis of measurable data, information or characteristics.

More specifically, it is a type of volumetric analysis that uses a solution of known volume and known concentration (primary standard solution) to accurately measure the unknown concentration of another solution.

Students should not be afraid of titrations as it is just one form of many quantitative analyses which they’ve already done before.

When studying for titration, students should focus not only on the calculations but also on memorising the detailed procedure of the technique.

Acid-Base Titration Summary

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It has already been stressed multiple times that accuracy is very important in titration. This means that not only the

volume, but also the concentration of the known solution must be accurate.

To ensure the accuracy of the concentration of the known solution, chemists create what’s known as a primary standard

solution.

is a solution of accurately known concentration that remains stable under

common laboratory conditions for extended periods of time.

is a chemical that can be made up into a solution of accurately known concentration.

To ensure that a primary standard fulfils its purpose, students should consider whether the primary standard:

has a large molar mass (hence, there is a smaller percentage error);

is cheap so that it is affordable for use;

is of high purity so that no side reactions occur;

is stable in the presence of air (so that no reaction with air occurs);

has any water of hydration, which may change due to atmospheric conditions such as humidity; and

dissolves readily in solvent (generally water) to form a stable solution.

Anhydrous sodium carbonate (𝑁𝑎2𝐶𝑂3) and sodium hydrogen carbonate (𝑁𝑎𝐻𝐶𝑂3) are frequently used as primary

standards in school science laboratories since they meet all the criteria.

is a solution that has its concentration measured by titration with a primary

standard solution. Therefore, when the concentration of the unknown solution in a titration is found, that solution

becomes a secondary standard solution which is then used to titrate another solution.

For example, acidic primary standard solution can be used to determine the concentration of a basic solution through

titration. Once the concentration is found, the basic solution can be used as the secondary standard solution to find

the concentration of an acidic solution.

Students should note that the accuracy of the concentration decreases every time this process is repeated because

secondary standard solutions tend to be more reactive and less pure than primary standard solutions.

For exams, students should focus on memorising the criteria used to choose an appropriate primary standard for acid-base titration as well as the procedures in preparing such primary standard solution.

Primary Standard Exam Tip

Primary Standard (known m) + Water (known V) = Standard Solution

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Before studying titration in more detail, students must learn some key words which are frequently used in discussing

acid-base titrations.

: the point at which the amounts of acid and base in the mixture solution are in

stoichiometric ratio of the balanced chemical equation (i.e. the point at which just enough acid or base has been

added to completely neutralise the unknown solution).

For better understanding, let’s consider the neutralisation of sulfuric acid with sodium hydroxide

𝐻2𝑆𝑂4(𝑎𝑞) + 2𝑁𝑎𝑂𝐻(𝑎𝑞) → 𝑁𝑎2𝑆𝑂4(𝑎𝑞) + 2𝐻2𝑂(𝑙)

The stoichiometric ratio of the acid (𝐻2𝑆𝑂4) to the base (𝑁𝑎𝑂𝐻) is 1: 2.

The equivalence point is the point at which the amount of the acid (in mol) and the amount of the base (in mol)

present in the reaction mixture is exactly in a 1: 2 ratio.

Since 1 mole of 𝐻2𝑆𝑂4 dissociates to produce 2 moles of hydrogen ions (𝐻+) and 2 moles of 𝑁𝑎𝑂𝐻 dissociates

to produce also 2 moles of hydroxide ions (𝑂𝐻−), the equivalence point can also be described as the point at

which the amounts and hence, the concentrations of 𝐻+ and 𝑂𝐻− in the reaction mixture is the same (i.e.

[𝐻+] = [𝑂𝐻−]).

: the point at which the acid-base indicator PERMANENTLY changes colour in a colourimetric

titration. The picture below depicts the reaction mixture of a titration before, at and after the end point.

Colourimetric (of colours) titration is the most common form of acid-base titration studied in the HSC. The name

comes from the fact that we are using the colour change as an indicative of the end point.

However, students must know that there are other types of indicators (i.e. non-colourimetric) which indicates the end

point not by the colour change but by the change in the conductivity, as will be covered in the later section in this

Theory Booklet.

Sample


Aim

To make a (primary) standard solution.

Background

An accurately measured mass of primary standard is dissolved in water in a volumetric flask to form an accurately

known volume of solution.

By knowing the mass of substance dissolved and the volume of the solution, it is possible to calculate the precise

concentration of the standard solution.

Apparatus

Risk & Management

Procedure

1. Rinse the volumetric flask with a small volume of distilled water.

2. Place a clean, dry 150 mL beaker on the electronic balance and tare (reset the balance to zero to measure only the

contents of the beaker) the balance.

3. Accurately measure out about 1.40 g of anhydrous sodium carbonate into the beaker and record exact mass.

4. Add about 80 mL of distilled water to the beaker and stir until the sodium carbonate has completely dissolved.

o 250 mL volumetric flask with lid

o Electronic balance

o Clean, dry 150 mL beaker

o Spatula

o Approximately 1.5 g anhydrous sodium carbonate

o 300 mL distilled water

o Wash bottle filled with distilled water

o Filter funnel

o Stirring rod

o Disposable droppers

o Safety glasses

Risk Management

Solid sodium carbonate can irritate skin and eyes.

Wear safety glasses and wash your hands at the end of the experiment.

Sample


5. Place the filter funnel into the neck of the volumetric flask.

6. Pour the sodium carbonate solution into the volumetric flask.

7. Use the wash bottle to wash the beaker 3 times around its inner side and add these washings to the volumetric

flask.

8. Rinse the rod and the filter funnel with water from the wash bottle. All these washings go into the volumetric flask.

9. Remove the funnel and fill the volumetric flask with distilled water until the bottom of the meniscus is just

touching the engraved line on the neck of the volumetric flask to avoid parallax error.

10. Stopper the flask and invert it to mix the contents thoroughly, while firmly holding the lid in place. The flask

should be inverted and shaken four or five times.

11. Calculate the actual concentration of the standard solution using the exact mass that was actually measured.

12. Label the flask with this concentration and formula.

Results

Accurately record the mass of anhydrous sodium carbonate that was measured out (e.g. 1.398 g)

Analysis

1. Calculate the concentration of the sodium carbonate solution, correct to 4 significant figures.

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

Conclusion

Primary standard solution of sodium carbonate was successfully prepared.

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Discussion

1. Justify why the beaker and filter funnel were rinsed after use.

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

2. Evaluate the accuracy of this technique for preparing a primary standard solution.

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

3. Sodium carbonate decahydrate (𝑁𝑎2𝐶𝑂3⦁10𝐻2𝑂) is readily available, has a higher molar mass and is cheaper than anhydrous sodium carbonate. Justify why anhydrous sodium carbonate is used as a primary standard, while sodium carbonate decahydrate is not.

…………………………………………………………………………………………………………………………………………………………………....

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…………………………………………………………………………………………………………………………………………………………………....

…………………………………………………………………………………………………………………………………………………………………....

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Accuracy

Students studying chemistry would have encountered the terms “validity”, “reliability” and “accuracy” often,

especially when performing practical analysis.

Diagrams of shooting targets such as the one below is often used to demonstrate what these three terms mean.

Of the three, accuracy is a very important concept in volumetric analyses.

Sample


refers to the closeness of a measured value to a standard or known value.

Quantitatively, the accuracy can be calculated as a percentage error by the following formula:

% 𝒆𝒓𝒓𝒐𝒓 =𝑬𝒙𝒑𝒆𝒓𝒊𝒎𝒆𝒏𝒕𝒂𝒍 𝒗𝒂𝒍𝒖𝒆 − 𝒕𝒉𝒆𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍 𝒗𝒂𝒍𝒖𝒆

𝒕𝒉𝒆𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍 𝒗𝒂𝒍𝒖𝒆× 𝟏𝟎𝟎

Accuracy of your results can be analysed and assessed by cross-examination against the true or accepted value or,

where this is not possible, against secondary sources such as the results obtained by other students performing the

same experiment, etc.

Accuracy of your results can be improved by using appropriate and more sensitive equipment. For example, using

a measuring cylinder yields more accurate results when measuring volumes than using a beaker.

There are equipment or glassware used specifically for titrations to ensure the accuracy of the results obtained.

There are also specific procedures students must follow when performing titrations to ensure the accuracy of the

results.

Such glassware and procedures to ensure accuracy for performing titrations are thoroughly covered in the following

section.

Accuracy and Sig. Fig. for Volumetric Glassware

Students should be well aware of how to deduce the number of significant figures for answering calculation

questions in the exam as follow.

Number of sig. fig. in the final answer should be equal to the minimum number of sig. fig. used in the

calculation.

Whole numbers ending with zero(s) used in the calculation should be omitted from the above consideration.

Students must be very careful when dealing with the volumetric glassware such as a pipette, burette or volumetric

flask especially for titration calculation questions.

Although the volumes are sometimes written less precisely as 25 mL by pipette or 500 mL in a volumetric flask,

the students must realise that these values are actually represented in 1 decimal place (owing to their 0.1 mL divisions)

and hence their corresponding sig. fig. must be 25.0 or 500.0 mL respectively.

Accuracy refers to closeness of a measured value to a standard or known value.

In an acid-base titration, specialised equipment must be used to ensure an accurate

measurement of the volumes.

For exams, students must be aware of the concept of significant figures and how it

is applied in calculations for titrations.

Accuracy Summary Sample


Volumetric analysis involves the measuring of volumes. Therefore, volumetric glassware must be used to ensure an accurate measurement of the volumes.

Volumetric Flask

A volumetric flask is a flat-bottomed volumetric glassware with a long narrow neck. A

line on the flask indicates the volume that can be accurately measured (diagram right).

It is often used in the preparation of the primary standard solution which will be

titrated against the analyte.

The volumetric flask is always rinsed with the solvent, generally water, prior to use. This

is an important step in titration and students will learn about it later.

The bottom of the meniscus must be just touching the line on the flask (to avoid

parallax error).

It is important not to overfill the flask, because then the volume of solution will not be

accurately known. If this happens, you will need to empty the flask and start again from

the beginning.

Burette A Burette is volumetric glassware which is used to deliver accurately measured volumes of liquid.

A burette is a long narrow glass tube with a tap at the bottom (diagram right). The most commonly used burette

is a 50 mL burette. They are usually graduated from 0.0 mL to 50.0 mL with

0.1 mL divisions.

The acid is usually placed in the burette since bases have the capacity to etch

(to cut into) the glass and, over time, change the volume of the burette.

However, normally it does not matter which solution is used in the burette as

it takes long period time for such etching effect to occur.

The measuring scale on a burette is inverted, since it measures the volume of

solution that leaves the burette through the tap (i.e. the zero reading is at the

top of the burette). Therefore, the burette is used to transfer an accurate

volume of the titrant to the analyte in the volumetric flask. The difference

between the initial and final readings on the burette indicates the volume of

solution delivered in a titration.

The burette is always rinsed with the solution that will be placed in the burette

prior to use.

To rinse the burette, pour a small volume of solution into the burette.

Then, holding the burette horizontally, open the tap and turn the burette slowly in your hands so that all

surfaces on the inside of the burette are rinsed with the solution.

After that, empty this solution.

To fill the burette, the following steps are taken.

Place the burette back in the burette clamp and close the tap.

Pour the solution into the burette until it is above the zero mark.

Place a flask under the burette and open the tap so that the airlock below the tap is filled with solution.

Adjust the volume of solution in the burette to the zero mark.

Sample


Pipette

A pipette is a volumetric glassware which is used to transport a specific (fixed) & accurately measured volume

of liquid (e.g. 25.00 𝑚𝐿) into a conical flask prior to titration. A line on the pipette indicates the volume that can

be accurately measured.

Unlike the burette, the pipette can only deliver a fixed volume of a solution. The most commonly used pipette in

school laboratories are 20 mL and 25 mL pipettes. This means that these pipettes can only deliver 20 mL and 25

mL of any solution.

The pipette is always rinsed with the solution that will be placed in it prior to its use (elaborated later).

To rinse the pipette, hold the pipette horizontally and turn the pipette in your hands so that all surfaces on the

inside of the pipette are rinsed with the solution. Then, empty this solution.

The pipette is usually used to transfer an accurate volume (usually 25 mL) of the

analyte into the volumetric flask.

To fill the pipette, use the pipette filler to draw the solution into the pipette.

Stop when the volume of solution is above the graduation mark.

Slowly evacuate the solution until the bottom of the meniscus is just

touching the mark.

Place the pipette over the flask into which the solution will be evacuated,

remove the pipette filler and allow the pipette to empty.

Tap the tip of the pipette on the inside of the flask. A small volume of solution will remain in the pipette. Do

not try to evacuate this solution. The pipette has been calibrated to evacuate the specified volume when it

empties naturally under gravity. The portion of liquid remaining in the tip of the pipette must not be blown

out.

Parallax Error

Because accuracy is very important when performing titration, students

must be careful not to commit parallax errors when reading the volumes.

is an error in measurement due to the observer’s

position relative to the measuring scale. If the reading is taken at an angle

to the scale, then an error is introduced (see diagram 1 on the right).

The term “parallax error” has been used when discussing volumetric

flasks. However, students should take care not to commit the error when

reading volumes in any volumetric glassware.

To minimise parallax error, students should ensure that the graduation

mark (engraved line) is directly parallel with their eye level.

Moreover, when water is placed in a thin tube such as the burette, the

capillary attraction between the water and the glassware creates a curve,

known as meniscus, in the upper surface of the liquid (see diagram 2).

When reading the volume, students must ensure that their eye level is

directly in line with the bottom of this meniscus .

If the meniscus is above eye level an increased volume measurement will

be made, conversely if the eye is above the meniscus then a lower volume

reading will be made.

○1

○2

Rubber pipette filler

Pipette

Sample


① Wash the burette with distilled water several times first (make sure to clean the space below the tap) and then

rinse it 3 times with small volumes of titrant (i.e. with the solution that is going to be placed in it). These washings are then discarded.

If the burette is rinsed with distilled water only, then water droplets remaining on the inside of the glassware will

dilute the solution and therefore the number of moles of solution can no longer be accurately determined.

② The burette is then clamped (using a burette clamp) to a retort stand.

③ A small, clean glass funnel is used to fill the burette with the titrant to above the zero mark. The tap is then opened while the titrant runs into a waste beaker until the base of the meniscus is on the zero line while avoiding a parallax error.

, just fill up the burette with the titrant to any level above 20mL and accurately read and record the solution level, then when the equivalence point is reached, read the final level of the solution. The difference between the final and initial levels in the burette is the volume of titrant (i.e. titre) used.

Ensure that the titrant has filled the space below the tap and that there are no bubbles present.

④ The pipette filler (i.e. the rubber pipetting ball) is used to draw up some distilled water to rinse the pipette several

times and these washings are discarded. The pipette is then rinsed with small volumes of the analyte and these washings are also discarded.

⑤ The pipette can then be filled with the analyte while ensuring the bottom of the meniscus to be on the engraved line. Wipe the outside of the pipette with a tissue.

If the pipette is rinsed with distilled water only, then water droplets remaining on the inside of the glassware will

dilute the solution and therefore the number of moles of solution can no longer be accurately determined.

⑥ The conical flask is thoroughly cleaned and rinsed several times with distilled water. It can be left wet, as water will be added anyway during the titration.

It is not necessary for the conical flask to be dry, as any water remaining in the flask will not change the number

of moles of solution they hold.

In other words, what is important in the conical flask is the number of moles of each reactant (i.e. titrant and

analyte).

⑦ The solution in the pipette is now transferred to the conical flask by draining it under gravity into the conical flask.

The tip of the pipette should rest against the inside glass wall of the flask so that adhesive forces ensure that the

liquid is fully transferred into the flask.

Do not shake out or blow out the remaining drop in the pipette, as this has been already calibrated.

⑧ A control is set up by repeating steps 5 to 7 in a separate conical flask in order to compare the changing colour of the solution to the original colour on order to more easily observe the colour change in later steps.

Sample


⑨ Add 2~3 drops of the suitable indicator to each of the two conical flasks which are placed on top of white tiles. Set one flask to act as a reference (i.e. the control) while placing the other flask under the burette.

⑩ Open the tap and add the titrant while using your other hand to continuously swirl the flask to mix the solutions during addition until a point is reached where the colour of the indicator in the flask just changes which indicates that the equivalence point is near.

⑪ Swirl the flask to mix the solutions well and check whether the colour change was temporary or permanent.

If permanent, just record the titre and repeat the whole method from the beginning as it means that you have just

overshoot the equivalence point.

If temporary, continue with the next step.

⑫ Carefully open the tap so that only one drop of the titrant is added and then swirl the flask to check whether there is a permanent colour change. If not, repeat this process until there is a permanent colour change.

If any of the solution splashes up onto the sides of the flask, use the wash bottle to rinse the liquid down into the

reaction mixture.

⑬ Record the volume of the titrant added from the burette.

⑭ Repeat the titration several times until a set of consistent volumes of titrant is obtained (volumes differing by no more than about ± 0.1 or ± 0.2 mL)

Note on the First (Rough) Titration

⊳ The first titration is usually called ‘rough’ titration as you will probably have missed the exact colour change or end-point because you do not know at what approximate volume it will occur. This is called overshooting the equivalence point.

⊳ The rough titration allows you to locate the approximate end-point so that in subsequent titrations you can approach the end-point more slowly, and obtain a more accurate result.

⊳ A skilled chemist can transfer fractions of drops from the burette tip into the reaction solution using a wash bottle.

Sample


Aim

To determine the concentration of a solution of hydrochloric acid using volumetric analysis, titration.

Apparatus

Risk & Management

Procedure

1. Rinse one of the 150 mL beakers with a small amount of the hydrochloric acid solution, empty it, label it and fill

with about 100 mL of the hydrochloric acid solution.

2. Rinse and fill the burette with the hydrochloric acid solution. Record the initial reading.

3. Rinse the other 150 mL beaker with a small amount of the sodium carbonate solution, empty it, label it and fill

with about 100 mL of the sodium carbonate solution.

4. Rinse the conical flask with water.

5. Rinse the pipette with sodium carbonate solution, then use the pipette to transfer 25.0 mL of the sodium

carbonate solution to the conical flask.

6. Add two drops of methyl orange indicator to the conical flask and swirl to mix.

7. Place the conical flask under the burette and slowly drop the acid into the conical flask.

8. When the first permanent colour change has occurred, record the reading the final reading on the burette.

9. Repeat the titration several more times and record your results.

o 250 mL of sodium carbonate primary standard solution (prepared in the previous experiment).

o 200 mL hydrochloric acid of unknown concentration

o 50 mL burette

o Retort stand and burette clamp

o 25 mL pipette and pipette filler

o 2 × 150 mL beakers

o 3 × 250mL conical flasks

o Dropper bottle containing methyl orange indicator

o 2 small labels (for the 150 mL beakers)

o Wash bottle with distilled water

o Filter funnel

o Safety glasses

Risk Management

Dilute solutions of hydrochloric acid, sodium carbonate and methyl orange indicator may splash onto your skin or into your eyes.

Wear safety glasses to avoid any splashes and wash your hands at the end of the investigation.

Glassware could break and cut your hands

Keep glassware away from edge of the bench. When pipette is not in use, leave pipette filler on pipette to prevent it rolling off the bench. If glassware does break, inform your teacher immediately.

Sample


Diagram of Experiment

Why do you Rinse with Different Solutions?

Rinsing the different equipment with different solutions is an important step to ensure the accuracy of the results

obtained in a titration.

Rinsing prevents the contamination of the solution which will be used to fill the different equipment. Different

glassware may contain different contaminants (e.g. remnants of other chemicals from previous uses, dust which

may have built up over time, etc.). These must be fully rinsed away to ensure the purity of the acid or base used in

an acid-base titration.

The burette and the pipette have been rinsed with the solutions which were subsequently used to fill those

apparatuses. This is because if these are rinsed with other solutions, the solution that is used to rinse can dilute or

even neutralise the acid or the base solution which will be filling in these volumetric glassware.

For example, if the pipette used in this titration was rinsed with water, droplets of water remaining after rinsing

can dilute the sodium carbonate solution subsequently added. In such case, the number of moles of sodium

carbonate in the delivered volume (say 25.0 mL) would be less than what it was supposed to be. This will have

detrimental effects on the accuracy of the results obtained, making the concentration found to be lower than

it should be due to the decrease in the concentration of the analyte.

Conversely, the conical flask has been rinsed with water only before it was filled with the sodium carbonate solution.

This is because in the conical flask, we care about having the exact number of moles of 𝑁𝑎2𝐶𝑂3 which will react

with the acid added from the burette.

If the conical flask is rinsed with sodium carbonate, the number of moles of 𝑁𝑎2𝐶𝑂3 that actually react with

the acid will be more than the amount used in the calculations due to the presence of droplets of 𝑁𝑎2𝐶𝑂3

remaining after rinsing. (i.e. more acid from the burette. will be required to completely neutralise the

𝑁𝑎2𝐶𝑂3). Conversely, if 𝐻𝐶𝑙 is used to rinse, the number of moles of 𝑁𝑎2𝐶𝑂3 that actually react with

the acid will be less.

Expected colour change of the reaction

mixture when enough acid is added

Sample


Results

Analysis

1. Calculate the average volume of hydrochloric acid used.

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2. Write a balanced chemical equation for the reaction between hydrochloric acid solution and sodium carbonate solution.

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3. Calculate the number of moles of 𝑁𝑎2𝐶𝑂3 placed in the conical flask.

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4. Use the mole ratio from your balanced chemical equation to determine the number of moles of hydrochloric acid that was titrated.

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5. Calculate the concentration of hydrochloric acid that was titrated

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The titration of ________ mL of _________________ acid with _________ mol/L ______________

Trial # 1 2 3 4 5 Average Titre

(mL) Initial Burette Volume (mL) 12.0 13.2 12.7 14.1 13.5

Final Burette Volume (mL) 30.4 27.4 26.7 30.5 27.6

Titre (mL) 18.4 14.2 14.0 16.4 14.1

Sample


Discussion

1. Discuss how the accuracy of your results could be improved.

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2. Justify why the conical flask was rinsed with water and not the solutions that were placed in it.

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Conclusion

The concentration of hydrochloric acid solution was accurately measured to be ________________ using titration.

Sample


Homework Exercise 1

1. Define the following terms: (a) volumetric analysis; (b) accuracy; (c) parallax error

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2. Describe how parallax error can be minimised.

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3. What is used to rinse a pipette? Explain why this is used instead of water.

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4. Marc wanted to fill a burette with hydrochloric acid. He poured some water into the burette, then opened the tap and emptied the burette. Marc then filled the burette with hydrochloric acid, adjusted the volume to the zero mark and indicated that he was ready to use the burette. Identify erros in Marc’s method and justify the correct method for using a burette.

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Sample


5. Identify three criteria that must be met for a substance to be used as a primary standard.

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6. Sodium hydroxide is hydrophillic (has a strong affinity for water). Explain why sodium hydroxide is not appropriate for use as a primary standard.

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7. You have been provided with standard solutions of the following primary standards:

A. Potassium hydrogen phthalate (𝐾𝐻(𝐶8𝐻4𝑂4))

B. Anhydrous sodium carbonate (𝑁𝑎2𝐶𝑂3)

C. Sodium borate (𝑁𝑎2𝐵4𝑂7 • 10𝐻2𝑂)

D. Hydrated oxalic acid (𝐻2𝐶2𝑂4 • 𝐻2𝑂)

Which one or more of the above solutions of these primary standards can be used in a titration to determine the concentration of a solution of

(a) Potassium hydroxide?

(b) Hydrochloric acid?

Justify your answer.

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Sample


8. Describe the purpose of a primary standard in an acid-base titration.

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9. Anhydrous sodium carbonate (𝑁𝑎2𝐶𝑂3 ) is used as a primary standard for the analysis of acids. Calculate the concentration of a standard solution prepared in a 50.00 mL volumetric flask by dissolving 13.25 g of anhydrous sodium carbonate in deionised water.

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10. Calculate the mass of potassium hydrogen phthalate, 𝐾𝐻(𝐶8𝐻4𝑂4), required to prepare 100.0 mL of a 0.200 molL-1 standard solution.

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11. Marc was making the following primary standard solution: 250 mL of a 0.135 molL−1 solution of sodium hydrogen carbonate.

(a) Calculate the mass of sodium hydrogen carbonate that must be used to make this solution.

(b) Describe the method used to make this primary standard.

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FREETRIAL LESSON

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