Question 18.13: Predicting Relative Acidity of Salt Solutions from Reactions......
Predicting Relative Acidity of Salt Solutions from Reactions of the Ions with Water
Problem Predict whether aqueous solutions of the following salts are acidic, basic, or neutral, and write an equation for the reaction of any ion with water: (a) Potassium perchlorate, KClO_4 (b) Sodium benzoate, C_6H_5COONa
(c) Chromium(III) nitrate, Cr(NO_3)_3
Plan The formula shows the cation and the anion. Depending on an ion’s ability to react with water, the solution will be neutral (strong-acid anion and strong-base cation), acidic (weak-base cation or highly charged metal cation with strong-acid anion), or basic (weak-acid anion and strong-base cation).
Solution (a) Neutral. The ions are K^+ and ClO_4^-. The K^+ is from the strong base KOH, and the ClO_4^- is from the strong acid HClO_4. Neither ion reacts with water.
(b) Basic. The ions are Na^+ and C_6H_5COO^−. The Na^+ is the cation of the strong base NaOH, so it does not react with water. The benzoate ion, C_6H_5COO^−, is the anion of the weak acid benzoic acid, so it reacts with water to produce OH^− ion:
C_6H_5COO^−(aq) + H_2O(l) \xrightleftharpoons[]{} C_6H_5COOH(aq) + OH^−(aq)
(c) Acidic. The ions are Cr^{3+} and NO_3^−. The NO_3^− is the anion of the strong acid HNO_3, so it does not react with water. The Cr^{3+} ion is small and highly charged, so the hydrated ion, Cr(H_2O)_6^{3+}, reacts with water to produce H_3O^+:
Cr(H_2O)_6^{3+}(aq) + H_2O(l) \xrightleftharpoons[]{} Cr(H_2O)_5OH^{2+}(aq) + H_3O^+(aq)