chemistry 3

ACID-BASE TITRATIONS Introduction:

According to the theory of Arrhenius,a substance that reacts with water to give hydronium ions,

H3O + , is an acid, and a substance that reacts with water to give hydroxide ions, OH

- is a base.

Some common acids and bases are given in Table 1. Acids react with bases to give salts, in

reactions that are called acid-base reactions. Some examples are given in Table 2. If the salt

formed is soluble under the conditions of the experiment, it remains in solution in the form of its

fully dissociated hydrated ions. Thus, the equation for the reaction of HCl (aq) with NaOH (aq)

HCl (aq) + NaOH (aq) NaCl (aq) + H2O

acid base salt

can also be written as

H3O + (aq) + Cl

- (aq) + Na

+ (aq) + OH

- (aq) Na

+ (aq) + Cl

- (aq) + 2H2O

We see that Na + (aq) and Cl

- (aq) appear on both sides of the equation and are spectator ions,

so that this acid-base reaction is more simply written as

H3O + (aq) + OH

- (aq) 2 H2O

This equation applies to the reaction of any strong acid with a strong base in aqueous solution.

Acids are classified as strong or weak, depending on whether or not they are completely

ionized in solution, for example,

HCl (aq) + H2O H3O + (aq) + Cl

- (aq) strong acid

HF (aq) + H2O H3O + (aq) + F

- (aq) weak acid

NaOH (aq) Na + (aq) + OH

- (aq) strong base

NH3 (aq) + H2O NH4 + (aq) + OH

- (aq) weak base

All acid-base reactions go to completion, due to removal of H3O + from the acid by OH

- from the

base. In the case of a weak acid, as H3O + is removed by OH

- , more of the undissociated acid

ionizes to give H3O + , until the reaction is complete. This is a simple application of Le Chatelier's

Principle.

Table 1

Some Common Acids and Bases

Acids Bases

Perchloric acid HClO4 (strong) Sodium hydride NaH (strong)

Hydrochloric acid HCl (strong) Sodium oxide Na2O (strong)

Nitric acid HNO3 (strong) Potassium hydroxide KOH (strong)

Phosphoric acid H3PO4 (weak) Calcium hydroxide Ca(OH)2 (strong)

Hydrofluoric acid HF (weak) Ammonia NH3 (weak)

Acetic acid CH3COOH (weak) Aniline C6H5NH2 (weak)

Table 2

Some Acid-Base Reactions

HCl (aq) + NaOH (aq) NaCl (aq) + HOH

HNO3 (aq) + KOH (aq) KNO3 (aq) + HOH

H2SO4 (aq) + NaOH (aq) NaHSO4 (aq) + HOH

H2SO4 (aq) + 2 NaOH (aq) Na2SO4 (aq) + HOH

2 H3PO4 (aq) + 3 Ca(OH)2 (aq) Ca3(PO4)2 + 3 HOH

HF (aq) + NH3 (aq) NH4F (aq) + HOH

CH3COOH (aq) + KOH (aq) CH3COOK (aq) + HOH

Titrations:

A common experimental procedure for following acid-base reactions in solution is to start with a

given volume of one reactant of unknown concentration in a conical flask, and to gradually add

to it a solution of known concentration of the other, until the reaction is complete. This

procedure is known as titration. A known volume of the first solution is introduced into the flask

using a pipet, and the second solution is gradually run into the flask from a buret, which is

graduated so that the delivered volume can be read off after successive additions.

Other types of reaction can be followed in the same way, including oxidation-reduction and

precipitation reactions.

Calculations:

In all acid-base reactions in aqueous solution, the fundamental premise is that at the

equivalence point exactly the same number of moles n of hydrogen ions (H + ) and hydroxide

ions (OH - ) have been added.

nH+ = nOH-

For a monoprotic acid A, such as potassium hydrogen phthalate, in order to determine the

number of moles nH+ of hydrogen ions that have been added, it is necessary to know either the

amount mA of the pure acid added (in grams) and the molar mass MMA of that acid (in grams

per mole) or the molar concentration MA of the acid solution (in moles per liter) and the volume

VA of that solution added (in liters)

nH+ = mA( 1 ) or nH+ = MA(VA)

MMA

Similarly for a monohydroxy base B, such as NaOH, the number of moles nOH- is determined by

nOH- = mB( 1 ) or nOH- = MB(VB)

MMB

Because potassium hydrogen phthalate is a solid acid that can be obtained in very pure form,

this acid is often used to prepare a standard solution of acid. A standard solution of potassium

hydrogen phthalate is conveniently prepared by weighing out the requisite mass of the solid,

and dissolving it in distilled water. This solution may then be used to determine the exact

concentration of a solution of sodium hydroxide, by means of an acid-base titration. (See the

following reaction.) We refer to this procedure as using the acid to standardize the NaOH

solution.

K + HC8H4O4

- (aq) + NaOH (aq) Na

+ K

+ C8H4O4

2- (aq) + HOH

This NaOH solution will then be used to determine the molar concentration of a solution of an

unknown solid acid as well as the molar mass of that acid.

EXPERIMENTAL PROCEDURE:

You will need approximately 500 mL of 0.1 M NaOH, which should be dispensed from the stock

reagent supply into a clean, dry 500 mL Erlenmeyer flask, fitted with a rubber stopper.

SAVE THIS SOLUTION TO TITRATE ALL ACID SAMPLES!

a) Standardization of NaOH (aq)

1. Accurately measure the mass of three approximately 0.6 g samples of potassium

hydrogen phthalate (KHP). Carefully transfer all of each sample into three separate

250 mL Erlenmeyer flasks. (Note that the samples need not have identical masses.)

Record all the masses on the data sheet. Dissolve each KHP sample in 25-50 mL

of distilled water.

2. Wash a 50 mL buret three times with distilled water. Rinse the buret three times with

approximately 10 mL samples of the NaOH solution. Finally, fill the buret with the NaOH

solution, making sure to eliminate any air bubbles from the buret tip. Record the

initial volume to two decimal places on the report sheet. The correct procedure for

reading the volume is to read the level of the bottom of the meniscus, making sure

that your eye is level with the bottom of the meniscus. If you have any doubts about

this procedure, have your instructor check your reading.

3. To the solution of KHP, add two drops of phenolphthalein indicator, and titrate your

sample with the NaOH solution until the first pink color which persists for 30 seconds.

While adding the NaOH solution from the buret, gently swirl the flask and

occasionally wash the sides down with distilled water from your wash bottle to ensure

that any NaOH (aq) which has splashed onto the sides reacts with the KHP solution.

After completion of the titration, record the final NaOH volume. Repeat the

titration using the second sample.

4. Calculate the molarity of the NaOH solution for each of the three samples titrated.

The three values should agree to within 0.5% of each other. Should you fail to

obtain this degree of precision, do a fourth titration or even a fifth, to obtain

this agreement.

b) Titration of the Unknown Acid

Obtain a sample of an unknown acid from your instructor; be sure to record the unknown

number.

1. Accurately measure the mass of approximately 0.2 g of your unknown sample.

Record all masses on the report sheet. Add 100 mL of distilled water to the sample.

If the sample does not dissolve completely, proceed with the titration (step 2), since

the acid should dissolve as it is titrated. In this case, make sure that all of the acid

has dissolved at the endpoint.

2. To the sample add 2-3 drops of phenolphthalein indicator and titrate with the

standardized NaOH solution to a faint pink color which persists for 30 seconds.

Remember to titrate slowly, to swirl the solution constantly during addition of

the base, and to wash down the sides of the flask occasionally. Check that all of the

solid acid has dissolved by the time that the endpoint is reached. Record

the initial and final buret readings for the sample on the report sheet.

3. Should the first titration require more than 50mL, or less than 10 mL of NaOH

solution, you will need to adjust the sample size for the second trial. If more than

50 mL of NaOH was used then use 1/2 of the mass that you used in trial one

for trial two. If less than 10 mL of NaOH was used, then use twice as much mass

as you used for sample one.

4. Repeat Steps 1 and 2 making the adjustments described in Step 3, if necessary.

After two titrations of your unknown acid, go to Step 5.

5. For each sample using the average value for the molarity of the NaOH solution,

calculate the molar mass of the solid acid, assuming that it is monoprotic (i.e., has one

ionizable H atom). If two results do not agree within 0.5%, repeat the titration until

this agreement has been achieved.

c) Identification of the Unknown Acid

We assume in the calculation of the molar mass of the unknown acid that it has one ionizable H

atom, but it is possible that there is more than one (i.e., it is a diprotic or triprotic acid).

Calculate the molar mass for each possibility, and, by comparison with the data given in Table

3, identify your unknown, which has been selected from this list. Complete the report sheet.

Table 3

Possible Unknown Solid Acids

Acid Molar Mass Number of Molecular Formula

(g/mole) Protons

Sulfamic acid 97.09 1 NH2SO3H

Potassium 204.23 1 KHC8H4O4

Hydrogen Phthalate

Succinic acid 118.10 2 HOOCCH2CH2COOH

Tartaric acid 150.10 2 OH OH

| |

HOOCCH-CHCOOH

Citric acid 192.15 3 CH2COOH

|

C(OH)COOH

|

CH2COOH

Date Sec. Name

Data and Calculations Sheet: Acid-Base Titrations

a) Standardization of NaOH (aq)

Data:

Sample 1 Sample 2 Sample 3

Mass of KHP (g)

Final buret reading (mL)

Initial buret reading (mL)

Volume of NaOH used (mL)

Calculations: (Show ALL calculations on a separate sheet)

Trial # Molarity of

NaOH (aq)

1. M

2. M

3. M

Average Molarity M

Date Sec. Name

b) Titration of Unknown Acid

Data:

Sample 1 Sample 2 Sample 3

Mass of unknown acid (g)

Final buret reading (mL)

Initial buret reading (mL)

Volume of NaOH used (mL)

Calculations: (Show ALL calculations on a separate sheet)

Trial # Molar Mass of

Monoprotic Acid

1. g/mol

2. g/mol

3. g/mol

c) Identification of the Unknown Acid

Average Calculated Molar Mass:

Monoprotic acid (HA) g/mol

Diprotic acid (H2A) g/mol

Triprotic acid (H3A) g/mol

Unknown #: __

Identity of Unknown Acid:

Date Sec. Name

ADVANCED STUDY ASSIGNMENT: Acid-Base Titrations

1. Calculate the molarity of each of the following solutions:

a) 10.00 mL of 0.500 M NaOH (aq), diluted to 250.0 mL of solution with

distilled water.

b) 0.800 g NaOH (s) dissolved in distilled water, to give 500.0 mL of solution.

2. What volume of each of the following acids would be required for complete

neutralization of 50.00 mL of 0.100 M NaOH?

a) 0.100 M HCl (aq)

b) 0.100 M oxalic acid, H2C2O4 (aq), (a diprotic acid)