JEE Main · 2025 · 04 Apr 2025 · Shift 2 · Equilibrium

A pH of $10$ gives pOH $=4$, so the hydroxide-ion concentration is $10^{-4}\mathrm{mol}{\mathrm{L}}^{-1}$, which in one litre is $10^{-4}\text{mol}$ of hydroxide. Each formula unit of magnesium hydroxide that dissolves completely furnishes two hydroxide ions, so the magnesium hydroxide required is $\frac{10^{-4}}{2}=5\times 10^{-5}\text{mol}$. With a molar mass of $58\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ this is $5\times 10^{-5}\times 58=2.9\times 10^{-3}\text{g}=2.9\text{mg}$, which to the nearest integer is $3\text{mg}$.