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Question 15.9: Calculate the molar solubility of BaSO4....

Calculate the molar solubility of \mathrm{BaSO}_{4}.

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Begin by writing the reaction by which solid \mathrm{BaSO}_{4} dissolves into
its constituent aqueous ions.

\mathrm{BaSO}_{4}(s)\rightleftarrows\mathrm{Ba}^{2+}(a q)+\mathrm{SO}_{4}^{2-}(a q)

Next, write the expression for K_{\mathrm{sp}}.

K_{\mathrm{sp}}=[\mathrm{Ba}^{2^{+}}][\mathrm{SO}_{4}^{2{-}}]

Define the molar solubility (S) as [{ B}a^{2+}]\ \mathrm{or}\ [\mathrm{SO}_{4}^{\ 2-}]  at equilibrium.

S=[Ba^{2+}]=[SO_{4}^{2-}]

Substitute S into the equilibrium expression and solve for it.

\begin{array}{c}{{K_{\mathrm{sp}}=[{ Ba}{}^{2+}][{ S}\mathrm{O}_{4}{}^{2-}]}}\\ {{={}S\times S}}\\ {{=S^{2}}}\end{array}

Therefore

S={\sqrt{K_{\mathrm{sp}}}}

Finally, look up the value of K_{\mathrm{sp}} in Table 15.2 and calculate S. The molar solubility of \mathrm{BaSO}_{4} is 1.{{0}}3\times10^{-5} mol/L.

\begin{array}{l}{{S=\sqrt{K_{\mathrm{sp}}}}}\\ {{=\sqrt{1.07\times10^{-10}}}}\\ {{=1.03\times10^{-5}\,\mathrm{M}}}\end{array}
Table 15.2 Selected Solubility-Product Constants (K_{sp})
Compound Formula K_{sp}
barium sulfate BaSO_4 1.07\times10^{-10}
calcium carbonate CaCO_3 4.96\times10^{-9}
calcium fluoride CaF_2 1.46\times10^{-10}
calcium hydroxide Ca(OH)_2 4.68\times10^{-6}
calcium sulfate CaSO_4 7.10\times10^{-5}
copper(II) sulfide CuS 1.27\times10^{-36}
iron(II) carbonate FeCO_3 3.07\times10^{-11}
iron(II) hydroxide Fe(OH)_2 4.87\times10^{-17}
lead(II) chloride PbCl_2 1.17\times10^{-5}
lead(II) sulfate PbSO_4 1.82\times10^{-8}
lead(II) sulfide PbS 9.04\times10^{-29}
magnesium carbonate MgCO_3 6.82\times10^{-6}
magnesium hydroxide Mg(OH)_2 2.06\times10^{-13}
silver chloride AgCl 1.77\times10^{-10}
silver chromate Ag_2CrO_4 1.12\times10^{-12}
silver iodide AgI 8.51\times10^{-17}

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