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)

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