Q25 / 90 · KVPY 2010 (SB/SX) · Chemistry

The ubility product of $\mathrm{Mg}(\mathrm{OH})_{2}$ is $1.0 \times 10^{-12}$. Concentrated aqueous $\mathrm{NaOH}$ ution is added to a $0.01 \mathrm{M}$ aqueous ution of $\mathrm{MgCl}_{2}$. The $\mathrm{pH}$ at which precipitation occur is-

  • A
    $7.2$
  • B
    $7.8$
  • C
    $8.0$
  • Correct answer
    D
    $9.0$

Solution

$\quad \mathrm{MgCl}_{2} \longrightarrow \mathrm{Mg}^{+2}+2 \mathrm{Cl}^{-}$ $$ \begin{array}{c} \quad ? \\ 0.01 \mathrm{M} \\ \mathrm{K}_{\mathrm{p}}=\mathrm{Q}=\left[\mathrm{Mg}^{+2}\right]\left[\mathrm{OH}^{-}\right]^{2} \\ 10^{-12}=[0.01]\left[\mathrm{OH}^{-}\right]^{2} \\ {\left[\mathrm{OH}^{-}\right]^{2}=10^{-10}} \\ {\left[\mathrm{OH}^{-}\right]=10^{-5}} \\ \mathrm{pOH}=5 \quad \therefore \mathrm{pH}=9 \end{array} $$

Hence, the correct answer is option: 'D'.

More Ionic Equilibrium Questions