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DETERMINATION OF K a VALUE OF A WEAK ACID

OBJECTIVE

The purpose of this experiment is to determine the acid ionization constant, Ka value of theunknown monoprotic acid solutions using two different experimental techniques.

INTRODUCTION

Determination of acidity in a substance or system is important in many situations, such as the

quality of drinking water, food preservation, and soil conditions for agriculture. Strength and

weakness of an acid is defined as its ability to donate protons to a base. The quantitative measure

of the strength of an acid is the acid ionization constant, K a. The ionization of a generic acid HA

can be determined by the above equation:

HA (aq) + H 2O (l) → H 3O+(aq) + A -

(aq) (Equation 1)

where HA is a generic acid that dissociates by splitting into A −, known as the conjugate base of

the acid, and the hydrogen ion or proton, H +, which, in the case of aqueous solutions, exists as a

solvated hydronium ion. The corresponding acid ionization constant expression, K a is generally

written as:

K a=[ ]

(Equation 2)

Two methods may be used to determine the K a value. Both methods require the use of a pH

meter. In the first method, a sample of acid is titrated with base. The pH values are plotted vs.

the volume of base added. The equivalence point is determined from the graph. Next, the

volume of base halfway to the equivalence point is found, and the pH at this volume is noted.

The [H +] corresponding to this pH is equal to the K a for the acid. At a point halfway to the

equivalence point, [H +] = [HA] = [A -] for a monoprotic acid.

The second method for determining K a values involves a “half volume” method. A

solution of the acid is prepared and divided in half as accurately as possible. One portion is

titrated to its endpoint with phenolphthalein. The two portions are then recombined, and the pH

of the resulting solution is measured.

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The characteristic of an acid is the K a value whereas it can be used to identify an unknown acid.

The K a is the value to indicate the strength of an acid. The larger the K a value, the stronger the

acid. The smaller the K a value, the weaker the acid.

strong acid: K a > 1

weak acid: K a < 1

An acid ionization constant, K a which is known as acidity constant is a quantitative measure of

the strength of an acid in solution. One measure of the strength of an acid is its ability to donate

protons to a base. It is the equilibrium constant for a chemical reaction known as dissociation in

the context of acid-base reactions.

HA A−

+ H+

The logarithmic constant, p K a, which is equal to −log 10 K a, is sometimes also (but incorrectly)

referred to as an acid dissociation constant:

pKa = pH = -log [Ka]

pK a> the smaller the extent of dissociation, contrast to pK a< the larger the extent of

dissociation.

THEORY

There are two general methods used to determine the K a value of a weak acid:

1. Titrating the weak acid with sodium hydroxide, 2. Measuring the pH of the weak acid solution

Part A : Determination of the K a value of weak acid by titration with NaOH

A titration curve is formed by plotting the pH of the acid solution against the volume ofNaOH added. The equivalence point of the titration is achieved when all of the weak acid (HA)has completely reacted with NaOH. On the titration curve, the equivalence point is read at themiddle of the region where the pH increases abruptly. The half-equivalence point for the titrationis achieved when one half of the base needed to complete to neutralize the acid has beenadded. The concentration of the acid in the solution, [HA] is equal to the concentration of itsconjugate base, [ ].

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[HA] = [ ] (Equation 3)

Equation 2 can be simplified to yield equation 4,

Ka = (Equation 4)

Taking the negative sign of the algorithm of each side of equation 4, we can derive theequation 5,

-log K a = -log

pKa = pH (Equation 5)

For equation 5, it indicates that the pK a for the acid is equal to the pH of the solution atthe half equivalence point. The K a of the acid is determined from the pK a value as follows:

Ka = 1 (Equation 6)

Part B : Determination of the K a from the initial concentration and pH of weak acid solution

From the pH of the acid solution (HA), it enables to determine the and ionconcentration, which is related to the pH of a solution by Equation 7,

= (Equation 7)

By substituting [HA], , and at equilibrium into equation 2, the value of Ka canbe calculated.

Acid Formula K a

Acetic acid CH 3COOH 1.8 X 10 -5 Benzoic acid C 6H5COOH 6.3 X 10 -5 Carbonic acid H 2CO 3 4.2 X 10 -7 Formic acid HCOOH 1.8 X 10 -4 Hypochlorous acid HOCl 3.5 X 10 -8 Dihydrogen phosphate ion H 2PO 4

- 6.2 X 10 -6 Hydrogen phosphate ion HPO 4

2- 3.6 X 10 -13 Hydrogen carbonate ion HCO 3

- 4.8 X 10 -11 Nitrous acid HNO 2 4.8 X 10 -11 Phenol C 6H6O 4.3 X 10 -4 Potassium Hydrogen Phthalate KC 8H5O 4 5.3 X 10 -6

Table 1: Ionization constant (Ka) for some weak acids at 25°C

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MATERIAL/APPARATUS

MATERIAL

0.1 M Sodium Hydroxide

0.1 M Unknown Weak Acid

APPARATUS

Distilled Water Burette and A Clamp

pH Meter Beakers

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Volumetric Pipette Conical Flask

Hot Plate Measuring Cylinder

Magnetic stirrer

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PROCEDURE

A. Determination of the K a value of the weak acid by titration with NaOH.

1. 40 mL of 0.1M unknown acid is approximately obtained.

2. 10 mL of the unknown acid was transferred to a 250 mL beaker using volumetric

pippete.

3. The titration process between unknown acid and NaOH in the burette was performed.

4. Steps 2-3 is repeated twice more.

5. The titration curve is drawn using Excel, and the volume of NaOH required to neutralised

the weak acid solution in each solution. The data is recorded.

6. The volume of NaOH added at the half equivalence point in each titration is calculated.

The weak acid solution is determined at the graph and the data is recorded.

7. The K a value for each titration is calculated.

B. Determination of the K a from the initial concentration and pH of a weak solution

1. 20 mL of the unknown weak acid solution (same as used in Part A) is added to a dry

beaker.

2. The pH electrode is removed from ph 7 buffer solution. The electrode is rinsed with

distilled water and the tip is dried of the probe.

3. The electrode is inserted into the beaker containing acid solution from step 1. The

solution is stirred. After the reading of pH is stabilized, the data is recorded.

4. The weak acid is decanted into the designated waste container.

5. Steps 1-4 is repeated twice more.

6. The pH probe is rinsed with distilled water.

7. The pH and the initial concentration of unknown acid are used to determine the K a .

8. The average K a for the weak acid is calculated and used in identifying the weak acid

from the table 1.