Upon completion of this topic, learners will be able to:

• demonstrate calculations that analyze amounts of acid and base dissociated.

We have seen that the magnitude of K_a indicates the strength of an acid. Another measure of the strength of an acid is its percent ionization, which is defined as:

The stronger the acid, the greater the percent ionization. For a monoprotic acid HA, the concentration of the acid that undergoes ionization is equal to the concentration of the H+ ions or the concentration of the A^- ions at equilibrium.

For example, a 0.035 M solution of HNO2 contains 3.7 × 10^-3 M H⁺ and its percent ionization is:

The extent to which a weak acid ionizes depends on the initial concentration of the acid. The more dilute the solution, the greater the percentage ionization.

Weak bases and base ionization constants

As we did with acids, we can measure the relative strengths of bases by measuring their base-ionization constant (Kb) in aqueous solutions. In solutions of the same concentration, stronger bases ionize to a greater extent, and so yield higher hydroxide ion concentrations than do weaker bases.

Water is the acid that reacts with the base, HB+ is the conjugate acid of the base B, and the hydroxide ion is the conjugate base of water. A weak base yields a small proportion of hydroxide ions.

In solving problems involving weak bases, you should follow the same principles as you followed for weak acids. The main difference is that we calculate [OH^–] first, instead of [H⁺].