The Foundations of Chemistry

A dilute solution of nicotinic acid was found to contain the following concentrations at equi-
librium at 25°C. What is the value of Ka? [HA]0.049 M; [H 3 O
][A]8.4 10 ^4 M.

Plan

We are given equilibriumconcentrations, and so we substitute these into the expression for Ka.

Solution

HA
H 2 O 34 H 3 O

A Ka

Ka1.4 10 ^5

The equilibrium constant expression is

Ka1.4 10 ^5 pKa4.85

You should now work Exercise 32.

EXAMPLE 18-8 Calculation of Kafrom Percent Ionization

In 0.0100 Msolution, acetic acid is 4.2% ionized. Calculate its ionization constant.

Plan

We write the equation for the ionization of acetic acid and its equilibrium constant expression.
Next we use the percent ionization to complete the reaction summary and then substitute into
the Kaexpression.

Solution

The equations for the ionization of CH 3 COOH and its ionization constant are

CH 3 COOH
H 2 O 34 H 3 O

CH 3 COO and Ka

Because 4.2% of the CH 3 COOH ionizes,

MCH 3 COOHthat ionizes0.0420.0100 M4.2 10 ^4 M

Each mole of CH 3 COOH that ionizes forms one mole of H 3 O
and one mole of CH 3 COO.
We represent this in the reaction summary.

CH 3 COOH 
H 2 O 34 H 3 O
 
 CH 3 COO

initial 0.0100 M 
 0 M 0 M

change 4.2 10 ^4 M 
4.2 10 ^4 M 
4.2 10 ^4 M

at equil 9.58 10 ^3 M 4.2 10 ^4 M 4.2 10 ^4 M

Substitution of these values into the Kaexpression gives the value for Ka.

Ka1.8 10 ^5

You should now work Exercise 38.

(4.2 10 ^4 )(4.2 10 ^4 )



9.58 10 ^3

[H 3 O
][CH 3 COO]



[CH 3 COOH]

[H 3 O
][CH 3 COO]



[CH 3 COOH]

[H 3 O
][A]



[HA]

(8.4 10 ^4 )(8.4 10 ^4 )



(0.049)

[H 3 O
][A]



[HA]

The structure of nicotinic acid is

Nicotinic acid, also called niacin, is

a necessary vitamin in our diets. It

is not physiologically related to

nicotine.

HC

N

CH

C

C

H

O

B

O

HC COOOH

18-4 Ionization Constants for Weak Monoprotic Acids and Bases 763

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 17.7, Determining Kaand Kb

Values.

Some common household weak

acids. A strip of paper impregnated

with a universal indicator is

convenient for estimating the

pH of a solution.