Question

The precipitate of $Ca{F}_2(K_{sp}=1.7\times {10}^{-10})$ is obtained when equal volumes of the following are mixed

• A. ${10}^{-4}M{Ca}^{2-}+{10}^{-4}M{F}^{-}$
• B. ${10}^{-2}M{Ca}^{2-}+{10}^{-3}M{F}^{-}$
• C. ${10}^{-5}M{Ca}^{2-}+{10}^{-3}M{F}^{-}$
• D. ${10}^{-3}M{Ca}^{2-}+{10}^{-5}M{F}^{-}$

Answer 1

The correct option is B ${10}^{-2}M{Ca}^{2-}+{10}^{-3}M{F}^{-}$

When ionic product is greater than solubility product, then precipitation occurs
$Ca{F}_2\rightleftharpoons {Ca}^{2+}+2F^{-}$
In (b) ionic product $=\left({10}^{-2}\right)\times {\left({10}^{-3}\right)}^2=1\times {10}^{-8}$
Here ionic product exceeds solubility product, hence precipitation occurs.