Question 17.SE.14: Which of these substances are more soluble in acidic solutio...
Which of these substances are more soluble in acidic solution than in basic solution: (a) Ni(OH)_2(s), (b) CaCO_3(s), (c) BaF_2(s), (d) AgCl(s)?
Analyze The problem lists four sparingly soluble salts, and we are asked to determine which are more soluble at low pH than at high pH.
Plan We will identify ionic compounds that dissociate to produce a basic anion, as these are especially soluble in acid solution.
Solve
(a) Ni(OH)_2(s) is more soluble in acidic solution because of the basicity of OH^-; the H^+ reacts with the OH^- ion, forming water:
\begin{aligned}Ni ( OH )_2(s) & \rightleftharpoons Ni ^{2+}(a q)+2 OH ^{-}(a q) \\2 OH ^{-}(a q)+2 H ^{+}(a q) & \rightleftharpoons 2 H _2 O (l) \\\hline \text{Overall:}Ni ( OH )_2(s)+2 H ^{+}(a q) & \rightleftharpoons Ni ^{2+}(a q)+2 H _2 O (l)\end{aligned}
(b) Similarly, CaCO_3(s) dissolves in acid solutions because CO_3^{2 -} is a basic anion:
\begin{aligned}CaCO _3(s) & \rightleftharpoons Ca ^{2+}(a q)+ CO _3^{2-}(a q) \\CO _3{}^{2-}(a q)+2 H ^{+}(a q) & \rightleftharpoons H _2 CO _3(a q) \\H _2 CO _3(a q) & \rightleftharpoons CO _2(g)+ H _2 O (l) \\\hline \text{Overall:} CaCO _3(s)+2 H ^{+}(a q) & \rightleftharpoons Ca ^{2+}(a q)+ CO _2(g)+ H _2 O (l)\end{aligned}
The reaction between CO_3^{2 -} and H^+ occurs in steps, with HCO_3^- forming first and H_2CO_3 forming in appreciable amounts only when [H^+] is sufficiently high.
(c) The solubility of BaF_2 is enhanced by lowering the pH because F^- is a basic anion:
\begin{aligned}BaF _2(s) & \rightleftharpoons Ba ^{2+}(a q)+2 F ^{-}(a q) \\2 F ^{-}(a q)+2 H ^{+}(a q) & \rightleftharpoons 2 HF (a q) \\\hline \text{Overall:}\quad BaF _2(s)+2 H ^{+}(a q) & \rightleftharpoons Ba ^{2+}(a q)+2 HF (a q)\end{aligned}
(d) The solubility of AgCl is unaffected by changes in pH because Cl^- is the anion of a strong acid and therefore has negligible basicity.