JEE MAIN - Chemistry (2024 - 30th January Morning Shift - No. 25)
The $$\mathrm{pH}$$ at which $$\mathrm{Mg}(\mathrm{OH})_2\left[\mathrm{~K}_{\mathrm{sp}}=1 \times 10^{-11}\right]$$ begins to precipitate from a solution containing $$0.10 \mathrm{~M} \mathrm{~Mg}^{2+}$$ ions is __________.
Answer
9
Explanation
$$\begin{aligned}
& \text { Precipitation when } Q_{s p}=K_{s p} \\
& {\left[\mathrm{Mg}^{2+}\right]\left[\mathrm{OH}^{-}\right]^2=10^{-11}} \\
& 0.1 \times\left[\mathrm{OH}^{-}\right]^2=10^{-11} \Rightarrow\left[\mathrm{OH}^{-}\right]=10^{-5} \\
& \Rightarrow \mathrm{pOH}=5 \quad \Rightarrow \mathrm{pH}=9
\end{aligned}$$
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