Question

Soluble salts containing the ammonium ion (NH4+) can give acidic, neutral or basic solutions when dissolved in water. For any given salt that contains ammonium ion, describe how you would predict whether an aqueous solution of that salt would be acidic, neutral or basic.

Note: Kb for the weak base, NH3, is 1.8E-5.

Answer 1

For salts of weak acids with weak bases (such as ammonia), the pH of solution depends on the relative values of pKa and pKb.

(1) When the dissociation constants of the acid and base are equal, pKa = pKb and pH = 7. The solution will be neutral.

(2) When pKa > pKb, the acid is relatively weaker than the base, the solution will be alkaline and pH > 7.

(3) When pKa < pKb, the acid is relatively stronger than the base and the solution will be acidic with pH < 7.