Question

Calculate the simultaneous solubilities cations of $Ca{F}_2$ and $Sr{F}_2$. $K_{sp}$ for the two salts are $4\times {10}^{-11}$ and $2.8\times {10}^{-9}$ respectively.

Answer 1

(i) $Ca{F}_2=C{a}^{2+}+2F^{-}$

Let the solubility of $Ca{F}_2$ be $s$

$\therefore \left[C{a}^{2+}\right]\times [F^{-}{]}^2=K_{sp}=4\times {10}^{-11}$

$\Rightarrow s\times (2s{)}^2=4\times {10}^{-11}$

$\Rightarrow s=2.15\times {10}^{-4}$

$\therefore$ Solubility of $Ca{F}_2$ will be $2.15\times {10}^{-4}M$ .

(ii) $Sr{F}_2\rightleftharpoons \underset{s}{S{r}^{2+}}+\underset{2s}{2F^{-}}$

Let the solubility of $Sr{F}_2$ is $s$

$\therefore \left[S{r}^{2^{+}}\right]\times [F^{-}{]}^2=K_{sp}=2.8\times {10}^{-9}$

$\Rightarrow 4s^3=2.8\times {10}^{-9}$

$\Rightarrow s=8.9\times {10}^{-4}M$

$\therefore$ Solubility of $Sr{F}_2$ will be $8.9\times {10}^{-4}M$ .