##### J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 16.66

Calculate the solubility of Mg(OH)2 in a buffer solution that is 0.75 M NH3 and 0.50 M NH4Cl.

The solubility of magnesium hydroxide will depend upon the concentration of hydroxide present (hydroxide is a common ion between the buffer and the salt). Thus, the first step in the problem is to find the concentration of hydroxide in the buffer. Then, a typical solubility equilibrium problem can be solved. Assume 25 °C.

Buffer solution:

 NH4Cl(aq) NH4+(aq) + Cl–(aq) NH3(aq) + H2O(l) NH4+(aq) + OH–(aq) Initial 0.75 0.50 0 Change – x + x + x Equilibrium 0.75 – x 0.50 + x x

Approximation is allowed, so

x = [OH] = 2.7×10–5 M

Solubility of Mg(OH)2

The hydroxide ion concentration will not change because of the buffer, so

 Mg(OH)2(s) Mg2+(aq) + 2 OH–(aq) Ksp = [Mg2+]e[OH–]e2 = 1.8×10–11 Initial 0 2.7×10–5 Change + x + 0 Equilibrium x 2.7×10–5

1.8×10–11 = [x][2.7×10–5]2

x = molar solubility of Mg(OH)2 (in the buffer) = 0.025 M