Chemical and Ionic Equilibrium 2 Question 31
31. The precipitate of $\mathrm{CaF}{2},\left(K{\text {sp }}=1.7 \times 10^{-10}\right)$ is obtained, when equal volumes of which of the following are mixed?
$(1982,1 \mathrm{M})$
(a) $10^{-4} \mathrm{M} \mathrm{Ca}^{2+}+10^{-4} \mathrm{M} \mathrm{F}^{-}$
(b) $10^{-2} \mathrm{M} \mathrm{Ca}^{2+}+10^{-3} \mathrm{M} \mathrm{F}^{-}$
(c) $10^{-5} \mathrm{M} \mathrm{Ca}^{2+}+10^{-3} \mathrm{M} \mathrm{F}^{-}$
(d) $10^{-3} \mathrm{M} \mathrm{Ca}^{2+}+10^{-5} \mathrm{M} \mathrm{F}^{-}$
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Solution:
- For precipitation reaction, $Q_{\mathrm{IP}}>K_{\mathrm{sp}}$.
$$ \begin{aligned} Q_{\mathrm{IP}} & =\left[\mathrm{Ca}^{2+}\right]\left[\mathrm{F}^{-}\right]^{2}=\left(\frac{10^{-2}}{2}\right) \times\left(\frac{10^{-3}}{2}\right)^{2} \ & =1.25 \times 10^{-9}>K_{\mathrm{sp}}, \text { precipitate will be formed. } \end{aligned} $$
