Explanation

(a)
$As_{2}S_{3} \rightleftharpoons 2{As}^{3 +} + 3S^{2 -}$
5 mol/L $2s$ $3s$
$K_{sp} = \left\lbrack {As}^{3 +} \right\rbrack^{2}\left\lbrack S^{2 -} \right\rbrack^{3}$
$= (2s)^{2}(3s)^{3}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 108s^{5}$
$\therefore$ $s = \sqrt[5]{\frac{K_{sp}}{108}}\ \ \ \ or\ \ \ \left( \frac{K_{sp}}{108} \right)^{\frac{1}{5}}$
$= \sqrt[5]{\frac{2.8 \times 10^{- 72}}{108}}$
$= \sqrt[5]{25.93 \times 10^{- 75}}$
$= 1.92 \times 10^{- 15}$ mol/L