Question 16.22: Predict whether a 0.10-M solution of each of the following s...

Predict whether a 0.10-M solution of each of the following salts will be basic, acidic, or neutral: (a) \text{LiI}, (b) \text{NH}_4\text{NO}_3, (c) \text{Sr(NO}_3)_2, (d) \text{KNO}_2, (e) \text{NaCN}.

Strategy Identify the ions present in each solution, and determine which, if any, will impact the \text{pH} of the solution.

Setup

(a) Ions in solution: \text{Li}^+ and \text{I}^–. \text{Li}^+ is a Group 1A cation; \text{I}^– is the conjugate base of the strong acid \text{HI}. Therefore, neither ion hydrolyzes to any significant degree.

(b) Ions in solution: \text{NH}_4^+ and \text{NO}_3^–. \text{NH}_4^+ is the conjugate acid of the weak base \text{NH}_3; \text{NO}_3^– is the conjugate base of the strong acid \text{HNO}_3. In this case, the cation will hydrolyze, making the \text{pH} acidic:

\text{NH}_4^+(aq) + \text{H}_2\text{O}(l) \rightleftarrows \text{NH}_3(aq) + \text{H}_3\text{O}^+(aq)

(c) Ions in solution: \text{Sr}^{2+} and \text{NO}_3^–. \text{Sr}^{2+} is a heavy Group 2A cation; \text{NO}_3^– is the conjugate base of the strong acid \text{HNO}_3. Neither ion hydrolyzes to any significant degree.

(d) Ions in solution: \text{K}^+ and \text{NO}_2^–. \text{K}^+ is a Group 1A cation; \text{NO}_2^– is the conjugate base of the weak acid \text{HNO}_2. In this case, the anion hydrolyzes, thus making the \text{pH} basic:

\text{NO}_2^–(aq) + \text{H}_2\text{O}(l) \rightleftarrows \text{HNO}_2(aq) + \text{OH}^–(aq)

(e) Ions in solution: \text{Na}^+ and \text{CN}^–. \text{Na}^+ is a Group 1A cation; \text{CN}^– is the conjugate base of the weak acid \text{HCN}. In this case, too, the anion hydrolyzes, thus making the \text{pH} basic:

\text{CN}^–(aq) + \text{H}_2\text{O}(l) \rightleftarrows \text{HCN}(aq) + \text{OH}^–(aq)