Chemistry 112

Weak Acids

In general, we can write the reaction of a weak acid HA in aqueous as:

HA(aq) + H2O(l) H3O+(aq) + A(aq)

The equilibrium constant for this reaction is

Ka is the acid dissociation equilibrium constant.

Ka is a quantitative measure of acid strength:

A larger Ka means a stronger acid. (Why?)


Ka is often measured by one of two methods:

1. Measurement of the pH of a solution of known initial concentration of weak acid.

The pH of a solution gives the hydronium ion concentration at equilibrium. Using this information, the initial concentration, and the stoichiometry of reaction allows evaluation of all of the required equilibrium concentrations.

2. Measurement of the per cent ionization of a solution of known initial concentration of weak acid.

% ionization, given the notation , is defined as

Again, this information and stoichiometry allows calculation of all equilibrium concentrations so that Ka can be evaluated.

Each of these methods is direct application of our typical methodology for treating equilibrium problems.


Examples

0.100 mole of HF is dissolved in 1.00 L of water at 25 oC. The pH at equilibrium was found to be 2.08. Evaluate Ka.


A 0.0100 M solution of HNO2 is found to be 19% ionized at equilibrium. Find Ka.


Which is the stronger acid, HF or HNO2?

The values of Ka range from 10–1 to 10–13 so a scale similar to pH is often used to report equilibrium constants:

pKa = –log Ka

So for HF and HNO2, the pKa values are:

HF pKa = –log Ka = –log(7.5×10–4) = 3.12

HNO2 pKa = –log Ka = –log(4.6×10–4) = 3.34

Smaller pKa means a stronger acid.

Acid ionization constants are reported in tables either as Ka or as pKa.