Question

$MY$ and $N{Y}_3$, two nearly insoluble salts, have the same $K_{SP}$ values of $6.2\times 10^{-13}$ at room temperature. Which statement would be true in regard to $MY$ and $N{Y}_3$ ?

• A. The molar solubility of MY in water is less than that of $N{Y}_3$
• B. The salts $MY$ and $N{Y}_3$ are more soluble in 0.5 M KY than in pure water
• C. The addition of the salt of KY to solution of $MY$ and $N{Y}_3$ will have no effect on their solubilities
• D. The molar solubilities of $MY$ and $N{Y}_3$ in water are identical

Answer 1

Answer: (A)

For MY
$K_{SP}=S^2$
$S=\sqrt{K_{SP}}$
$=\sqrt{6.2\times 10^{-13}}$
$=\sqrt{62\times 10^{-14}}$
$\approx 8\times 10^{-7}$
for $N{Y}_3$,
$K_{SP}=27S^4$
$N{Y}_3\rightleftharpoons \underset{\text{ S}}{N^{+3}}+\underset{\text{ 3S}}{3Y^{-}}$
$S={\left(\frac{6.2\times 10^{-13}}{27}\right)}^{1/4}={\left(0.2296\times 10^{-13}\right)}^{1/4}$
$S=3.89\times 10^{-4}$