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17 Additional Aspects of Solutions to Exercises Aqueous Equilibria y(1.969 y+y) = 3.2 2.969 = = 3.283 X = 10⁻⁴ [Pb²⁺] = 6.5 10⁻⁴ M, [Sr²⁺] = 3.3 M, [SO₄²⁻] = (3.283 + 6.464) 10⁻⁴ = 9.7 X 10⁻⁴ M 17.104 MgC₂O₄(s) If [Mg²⁺] is to be 3.0 = 8.6 10⁻² = X 10⁻³ = 2.9 M The oxalate ion undergoes hydrolysis: 1L Kb = = 1.0 10⁻¹⁴ = 1.56 10⁻¹⁰ = 1.6 10⁻¹⁰ 17.105 The student failed to account for the hydrolysis of the ion. If there were no hydrolysis, [Mg²⁺] would indeed be 1.5 times that of [AsO₄³⁻]. However, as the reaction + 11 + proceeds, the ion product falls below the value for More Mg₃(AsO₄)₂ dissolves, more hydrolysis occurs, and so on, until an equilibrium is reached. At this point [Mg²⁺] in solution is much greater than 1.5 times free [AsO₄³⁻]. However, it is exactly 1.5 times the total concentration of all arsenic-containing species. That is, [Mg²⁺] + 17.106 Zn(OH)₂(s) 11 Zn(OH)₄²⁻(aq) 1L If 0.015 mol Zn(OH)₂ dissolves, 0.015 mol should be present at equilibrium. [OH⁻]² = (0.015) 138 ; 10⁻² M; ≥ 1.0 X 10⁻² M or pH ≥ 12.02 544