Question

$A_{3}B_{2}(s) \rightleftharpoons \underset{\frac{3x}{M}}{3A^{2 +}} + \underset{\frac{3x}{M}}{2B^{3 -}}\left( \text{Solubility =}\frac{x}{M}\text{ mol/l} \right)$

$K_{sp} = \left\lbrack A^{2 +} \right\rbrack^{3}\left\lbrack B^{3 -} \right\rbrack^{2} = \left( \frac{3x}{M} \right)^{3}.\left( \frac{2x}{M} \right)^{2} = \frac{108x^{5}}{M^{5}} $$$\therefore\frac{\left\lbrack B^{3 -} \right\rbrack}{K_{sp}} = \frac{2x/M}{108x^{5}/M^{5}} = \frac{M^{4}}{54x^{4}}$$