Q53 / 160 · JIPMER 2005 · Chemistry

Which has minimum solubility?

  • A
    $\mathrm{Bi}_2 \mathrm{~S}_3$
  • Correct answer
    B
    $\mathrm{Ag}_2 \mathrm{~S}$
  • C
    CoS
  • D
    PbS

Solution

Solubilities of salts are as follows : $\begin{aligned} \mathrm{Bi}_2 \mathrm{~S}_3 & =\sqrt[5]{\frac{K_{s p}}{108}} \\ & =\sqrt[5]{\frac{1.6 \times 10^{-72}}{108}}=1.7 \times 10^{-15} \\ \mathrm{Ag}_2 \mathrm{~S} & =\sqrt[3]{\frac{K_{s p}}{4}}=\sqrt[3]{\frac{1.6 \times 10^{-49}}{4}} \\ & =3.4 \times 10^{-17} \\ \mathrm{PbS} & =\sqrt{K_{s p}}=\sqrt{1.1 \times 10^{-29}} \\ & =3.3 \times 10^{-15} \\ \mathrm{CoS} & =\sqrt{K_{s p}}=\sqrt{2 \times 10^{-27}}=4.5 \times 10^{-14}\end{aligned}$ Hence, $\mathrm{Ag}_2 \mathrm{~S}$ is least soluble. So minimum solubility is of $\mathrm{Ag}_2 \mathrm{~S}$ as its force of attraction is greatest.

Hence, the correct answer is option: 'B'.

More Ionic Equilibrium Questions