experiment no. 8 acid-base titration

Experiment No. 8

Acid-Base Titration

De Jesus, Nicolle

Dela Cruz, Maria Margarita

Dela Cruz, Paula Mae

Dongon, Marinelle

1NUR-3

Group 4

Introduction

The reaction between a base and an acid involves neutralization. It can be

written as:

Acid + Base Salt + H2O

This reaction is used in titration, a quantitative analytical method. The objective is to

determine the concentration of an acid or base. In acid-base titration, the analyte is

made to react with the standard solution. The analyte is a measured volume of unknown

solution. On the other hand, the standard solution is a solution whose exact

concentration is known. The analyte is placed in an Erlenmeyer flask and the standard

solution is gradually added from a burette until complete neutralization has been

reached.

The equivalence point is the point at which all the acid (or base) in the sample has

completely reacted with the standard solution of base (or acid). The equivalence point is

also the endpoint of titration. The endpoint is indicated by the change in colour of an

appropriate indicator.

The appropriate indicator for titration depends on the strength of the acid and the

base reacting. For titrations of strong acids and bases, the pH at the endpoint is 7.0.

The indicator should change its hue at about pH 7.0. For weak acids with strong bases,

the indicator should change colour at a pH above 7.0. On the other hand, strong acids

with weak bases need an indicator that changes colour below 7.0.

Upon reaching the endpoint, the number of equivalents of the acid is equal to the

number of equivalents of the base. Equivalents are the quantities of substances that

have the same combining capacity in chemical reactions. In a neutralization reaction, it

is the mass of acid that yields 1 mol of H+ or the mass of base that reacts with 1 mol of

H+. At the endpoint of titration,

VaNa = VbNb

Where:

Va = Volume of an acid Vb = Volume of a base

Na = Normality of an acid Nb = Normality of a base

Normality is the number of equivalents of a solute per liter of solution.

N = no. Of equivalents of Solute Volume of Solution in Liters

Since the no. Of equivalents is equal to the grams of solute/gram-equivalent weight

(GEW),

N = g of solute GEW (Lsolution)

GEWacid = molar mass of the acid / no. Of replaceable H+

GEWbase = molar mass of the base / no. Of OH- ions

If the given concentration of the standard solution is normality, the concentration of the

unknown solution can be solved for either in one of the following: Normality, Molarity,

and Percentage. For this experiment, the percent concentration of the unknown solution

will be determined.

Objectives

Students should be able to perform an acid-base titration.

They should be able to use the gathered data to solve for the

concentration of an unknown solution.

The students should be able to work efficiently and use critical thinking.

Observations/Results

A. Analysis of commercial Vinegar 1 (Datu Puti) – Group 5

Specific Objective: To determine the concentration of vinegar by performing an

acid-base titration.

NaOH: 0.1229 N

Trial 1 Trial 2

Final Reading 47.2 mL 46.6 mL

Initial Reading 0 33.4 mL

Volume NaOH used: 47.2 mL 13.2 mL

Sample Used: Vinegar

Trial 1 Trial 2


Initial Reading 0 mL 8.5 mL

Volume Sample used: 8.5 mL 6 mL

Weight of Acid Sample .35 .34

%w/v of Acid in Sample 4.12 5.67

Mean % w/w: 4.9%

B. Analysis of commercial Vinegar 2 (Silver Swan) – Group 1

Specific Objective: To determine the concentration of vinegar through acid-base

titration.

NaOH: 0.1229 N

Trial 1 Trial 2



Volume NaOH used: 29.2 mL 30.7

mL

Sample Used: Silver Swan Vinegar

Trial 1 Trial 2

Final Reading 45mL 40mL

Initial Reading 50mL 45mL

Volume Sample used: 5mL 5mL

Weight of Acid Sample 0.22 g 0.23 g

%w/v of Acid in Sample 4.4 % 4.6%

Mean % w/v: 4.5%

C. Analysis of commercial Fruit juice – Group 2

Specific Objective: To determine the concentration of fruit juice by performing an

acid-base titration.

NaOH: 0.1229 N

Trial 1 Trial 2



Volume NaOH used: 3.5 mL 2.9 mL

Sample Used: Fruit juice

Trial 1 Trial 2

Final Reading 35 mL 45 mL

Initial Reading 25 mL 35 mL

Volume Sample used: 10 mL 10 mL


%w/v of Acid in Sample 0.28% 0.23%

Mean %w/v: 0.26%

D. Determining the Strength of Antacid

Specific Objective: To be able to observe changes in acid-base titration and

gather data to determine the concentration of an Antacid

Sample Used: NaHCO3

No. Of mg: 300 mg

HCl: 0.1141 N (For Mefenamic Acid: NaOH: _____ N)

Trial 1 Trial 2

Final Reading 30.9 mL 25 mL

Initial Reading 4 mL 0 mL

Volume used: 0.0269 L 0.025 mL

NaOH: 0.1229 N (For Mefenamic Acid: _____ N)

Trial 1 Trial 2

Final Reading 31.2 mL 7 mL


Volume used: 3.2 x 10 -3 L 3.5 x 10 -3 L

Equiv. Of Base/Acid (step

2)

3.07 x 10-3 2.85 x 10-3

Equiv. Of Base/Acid Not in

tablet (step 3)

3.9 x 10-4 4.3 x 10-4

Equiv. Of Base/Acid in

tablet (step 2 – step 3)

2.68 x 10-3 2.42 x 10-3

Weight of Base/Acid

Sample

0.3g 0.3 g

%w/w of Base/Acid in

Sample

75.04% 67.76%

Mean % w/w: 71.4%

E. Determining the Strength of Mefenamic Tablets:

Specific Objective: To be able to observe changes in acid-base titration and

gather data to determine the concentration of a Mefenamic tablet.

Sample Used: Generic Mefenamic acid tablet

No. Of mg: 500 mg

HCl: 0.1141 N (For Mefenamic Acid: NaOH: _____ N)

Trial 1 Trial 2


Initial Reading 24.8 mL 2.5 mL

Volume used: 15.2 mL 16.9 mL

NaOH: 0.1229 N (For Mefenamic Acid: _____N)

Trial 1 Trial 2


Initial Reading 3.7 mL 0.0 mL

Volume used: 25.1 mL 24.8 mL

Equiv. of Base(step 2) 3.08 x 10-3 3.05 x 10-3

Equiv. of Acid Not in tablet

(step 3)

1.73 x 10-3 1.93 x 10-3

Equiv. of Acid in tablet (step

2 – step 3)

1.35 x 10-3 1.12 x 10-3


%w/w of Acid in Sample 65.07 % 53.98 %

Mean % w/w: 59.53%

Discussion

Vinegar (CH3COOH)

The volume of the NaOH and vinegar used was obtained by subtracting the

initial reading in the burette from the final reading. The weight of the acid

sample is calculated by multiplying the Normality of the base to the Volume of

Base in Liters and to the GEW (gram-equivalent weight). The % m/v of acid in

sample was obtained by dividing the mass of the sample to the volume

sample in millilitres and multiplying it to 100. In getting the mean % w/v, we

added the two percents and divided them to two.

Antacid (NaHCO3)

An antacid tablet was weighed and recorded as 300 mg. It was placed in an

Erlenmeyer flask. HCl, the titrant, was added. The amount was recorded as

0.0269 L. The antacid was made to dissolve with the titrant using a stirring

rod. The stirring rod was rinsed within the flask. Two drops of the indicator

was mixed in. The solution should be permanently colourless by now. If a pink

color developed, more HCl should be added. The added HCl should be

recorded. The number of equivalents of excess titrant was then calculated to

be 3.07x10-3 by multiplying the volume in liters by the normality of HCl which

is 0.1141 N. The excess titrant or acid was then neutralized using a

backtitrant NaOH and the recorded amount was 3.2x10-3. It was added until a

permanent faint pink color was evident. The number of equivalents of the

backtitrant was calculated as 3.9x10-4 by multiplying the amount of backtitrant

with the normality of NaOH which is 0.1229 N. % mass was then calculated

by subtracting Eq of titrant minus Eq of backtitrant. This was then multiplied

by GEW of NaHCO3 which is 84. Dividing it by the mass of the tablet in g

which is 0.3 g and multiplying it by 100, the answer is 75.04% Another trial

was then made and the calculated %mass was 67.76%. The mean of these is

71.4%

Mefenamic Acid

A generic mefenamic acid tablet was weighed and recorded as 500 mg. It

was placed in an Erlenmeyer flask. Sodium Hydroxide (NaOH), the titrant,

was added. The amount was recorded as 0.0251 L. It was made to dissolve

with the titrant using a stirring rod. The stirring rod was rinsed within the flask.

Two drops of the phenolphthalein indicator was mixed in. The solution should

be permanently pink by now. If a pink color developed, more NaOH should be

added. The added NaOH should be recorded. The number of equivalents of

excess titrant was then calculated to be 3.08x10-3 by multiplying the volume in

liters by the normality of NaOH which is 0.1229 N. The excess titrant or base

was then neutralized using a backtitrant HCl and the recorded amount used

was .0152L . It was added until a permanent light pink color was evident. The

number of equivalents of the backtitrant was calculated as 1.73x10-3 by

multiplying the amount of backtitrant with the normality of HCl which is 0.1141

N. % mass was then calculated by subtracting Eq of titrant minus Eq of

backtitrant. This was then multiplied by GEW of mefenamic acid which is 241.

Dividing it by the mass of the tablet in g which is 0.5 g and multiplying it by

100, the answer is 65.07% Another trial was then made and the calculated

%mass was 53.98%. The mean of these is 59.53%

Study Guide Questions and Answers

1. Discuss briefly why the following procedure must be observed during titration:

a. Acids are placed in burettes with glass stopcocks while bases are placed in

rubber-tipped burettes.

Acids are placed in burettes with glass stopcocks to prevent corrosion.

Acids corrode rubber. On the other hand, bases are placed in rubber-

tipped burettes to prevent formation of sticky substance due to reactions

of glass and a base.

b. The burette is rinsed with the solution to be placed in it after rinsing with

distilled water.

This is done to prevent the solution from getting stuck to the sides of the

burette.

c. Before the initial reading is taken, the glass tips of the burette should be

completely filled with the solution.

The glass tips should be completely filled because the liquid that it will

hold is also included in the measuring the acid and taking its initial

reading.

d. Care should be taken not to spill any liquid from the flasks or burettes before

the end point is reached.

Care should be taken to prevent spilling of any liquid because it will have a

greatly affect the weight and concentration of the liquids present. The

experiment, therefore, cannot be considered accurate.

e. Rinsing the sides of the Erlenmeyer flask with the distilled water before the

end point is reached.

We rinse the sides of the Erlenmeyer flask so all the analyte will react with

the standard solution.

2. What is the effect of the following condition on the calculated percentage of the

acid sample? (Increased, decreased or No effect at all).

a. The glass tips of the burettes were not filled with the sample solutions when

the titration was started.

Decreased. It’s because when the NaOH decreases, the percentage of

acid will decrease.

b. More distilled water was added to the flask than what was required in the

procedure.

No effect because distilled water is neutral that’s why it won’t affect the

acidity nor the basicity of the sample solution.

c. The base burette was not rinsed with the standard base before filling it with

the standard solution.

No effect because the basic solution and the acidic solution will not mix.

d. The standard base was allowed to flow down the sides of the flask leaving

some drops unreacted until the endpoint is reached.

Increase because an increase of NaOH will increase the percentage of

acid sample.

e. Just before the end point is reached, the sides of the flask were rinsed with is

much distilled water that droplets of the standard solution adhering it the side

combined with the solution being titrated.

Increased. The droplets of NaOH mixed with the acid sample making it

more basic.

3. Write the general equation for an acid-base titration.

Acid + Base Salt + H2O

4. Describe the action of mefenamic acid and NaHCO3 in our body.

Mefenamic acid works by blocking the action of a substance in the body

called cyclo-oxygenase (COX). Cyclo-oxygenase is involved in the production of

various chemicals in the body, some of which are known as prostaglandins.

Prostaglandins are hormones produced by the body in response to injury and

certain diseases and conditions, and cause pain, swelling and inflammation.

Mefenamic acid blocks the production of these prostaglandins and is therefore

effective at reducing inflammation and pain. Mefenamic acid may also work by

preventing the action of prostaglandins after they have already been formed.

Sodium Bicarbonate (NaHCO3) is used as an antacid to treat acid indigestion and

heartburn. An aqueous solution can be administered intravenously for cases of acidosis

(low pH), or when there are insufficient sodium or bicarbonate ions in the blood. Here’s

an example of its chemical reaction:

HCl+NaHCO3 NaCl+CO2+H2O

5. The electrolyte in body fluids is measured in terms of an equivalent at

milliequivalent. Define an equivalent of an ion. Give the formula needed to

calculate the equivalent weight of an ion.

An equivalent (Eq) of an ion is the molar amount of that ion needed to

supply one mole of positive or negative charge.

GEWacid = molar mass of the acid / no. Of replaceable H+

GEWbase = molar mass of the base / no. Of OH- ions

Eq weight= molar mass/ionic valence

6. What is the relationship between an equivalent and a milliequivalent. How is a

concentration of milliequivalents per liter converted to millimoles per deciliter?

Equivalent (Eq) of an ion is the molar concentration of that ion needed to

supply one mole of (+) or (-) integer. A small unit is used in specifying electrolyte

concentrations, which is the milliequivalent unit. The reason why is because of

the relatively low concentrations of ions present in body fluids.

1 milliequivalent = 10-3 equivalent

7. One of the cations present in the body is Ca2+. How many milliequivalents of Ca2+

are present in 100 ml of a 0.1% (w/v) Ca2+ solution?

8. In body fluids, concentrations of electrolytes are expressed as mEq.L. A patient’s

chart shows that his serum chloride ion concentration is 94 mEq.L. What is this

concentration on millimoles/liter.

94 mEq/L x 1Eq/1000mEq x 1 mole/ 1 Eq x 1000 mmol/ 1 mole = 94 mmol/L

9. Solve the following problem: Aspirin contains a monoprotic called acetylsalicylic

acod. Its molecular formula is C9H8O4. A certain pain reliever was analyzed by

dissolving 0.250 g of the sample in 20.0 mL of 0.10 N NaOH and bactitrating the

excess base using 0.120 N HCl solution. The backtitration process required 805

mL of HCl. What is the 5 by mass of acetylsalicylic acid in the pain reliever?

References

1. Chemical Technician’s Ready Reference Handbook. Third Edition. (Shugar &

Ballinger) – pp. 605-610, 620-627

2. A laboratory for General, Organice, and Biochemistry. Fourth Edition.

(Henrickson, Byrd, & Hunter) – pp. 172-181

3. Chemistry: The Molecular Nature of Matter and Change. Fourth Edition.

(Silberberg) – pp. 144-149

4. Exploring General, Organic, and Biological Chemistry. Philippine Edition. (Stoker)

– pp. 376-379

experiment no. 8 acid-base titration

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