Question 17.8: Calculating Molar Solubility from Ksp Calculate the molar so...
Calculating Molar Solubility from K_{sp}
Calculate the molar solubility of PbCl_2 in pure water.
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Begin by writing the reaction by which solidPbCl_2dissolves into its constituent aqueous ions and write the corresponding expression for K_{sp}.
PbCl_2(s) \rightleftharpoons Pb^{2+}(aq) + 2 \ Cl^-(aq)
K_{sp} = [Pb^{2+}][Cl^-]^2
Refer to the stoichiometry of the reaction and prepare an ICE table, showing the equilibrium concentrations of Pb^{2+} and Cl^- relative to S, the amount of PbCl_2 that dissolves.
PbCl_2(s) \rightleftharpoons Pb^{2+}(aq) + 2 \ Cl^-(aq)
[Pb^{2+}] | [Cl^−] | |
Initial | 0.00 | 0.00 |
Change | +S | +S |
Equil | S | 2S |
Substitute the equilibrium expressions for [Pb^{2+}] and [Cl^−] from the previous step into the expression for K_{sp}.
K_{sp} = [Pb^{2+}][Cl^-]^2
= S(2S)^2 = 4S^3
Solve for S and substitute the numerical value of K_{sp} (from Table 17.2) to calculate S.
S=\sqrt[3]{\frac{K_{sp}}{4} }
S=\sqrt[3]{\frac{1.17 \times 10^{-5}}{4} } = 1.43 \times 10^{-2 } \ M
TABLE 17. 2 Selected Solubility-Product Constants (K_{sp}) at 25 °C | |||||
Compound | Formula | K_{sp} | Compound | Formula | K_{sp} |
Barium fluoride | BaF_2 | 2.45 × 10^{-5} | Lead(II) chloride | PbCl_2 | 1.17 × 10^{-5} |
Barium sulfate | BaSO_4 | 1.07 × 10^{-10} | Lead(II) bromide | PbBr_2 | 4.67 × 10^{-6} |
Calcium carbonate | CaCO_3 | 4.96 × 10^{-9} | Lead(II) sulfate | PbSO_4 | 1.82 × 10^{-8} |
Calcium fluoride | CaF_2 | 1.46 × 10^{-10} | Lead(II) sulfide* | PbS | 9.04 × 10^{-29} |
Calcium hydroxide | Ca(OH)_2 | 4.68 × 10^{-6} | Magnesium carbonate | MgCO_3 | 6.82 × 10^{-6} |
Calcium sulfate | CaSO_4 | 7.10 × 10^{-5} | Magnesium hydroxide | Mg(OH)_2 | 2.06 × 10^{-13} |
Copper(II) sulfide* | CuS | 1.27 × 10^{-36} | Silver chloride | AgCl | 1.77 × 10^{-10} |
Iron(II) carbonate | FeCO_3 | 3.07 × 10^{-11} | Silver chromate | Ag_2CrO_4 | 1.12 × 10^{-12} |
Iron(II) hydroxide | Fe(OH)_2 | 4.87 × 10^{-17} | Silver bromide | AgBr | 5.36 × 10^{-13} |
Iron(II) sulfide* | FeS | 3.72 × 10^{-19} | Silver iodide | AgI | 8.51 × 10^{-17} |
*Sulfide equilibrium is of the type: MS(s) + H_2O(l)\rightleftharpoons M^{2+}(aq) + HS^-(aq) + OH^-(aq)