Which amongst the following aqueous solutions of electrolytes will have minimum elevation in boiling point? Choose the correct option.

Solution & Explanation

\textbf{Answer:} (C) 0.05 M $\mathrm{NaCl}$ \textbf{Solution:} Elevation of boiling point is a colligative property: $\Delta T_b=i{K}_{b}m$, so it depends on the effective particle concentration $i\times m$. Compute $i\times m$ for each (using molarity ≈ molality for dilute aqueous solutions): $\mathrm{MgSO}{}_4$ (0.1 M, $i=2$): $0.1\times 2=0.2$. $\mathrm{NaCl}$ (1 M, $i=2$): $1\times 2=2.0$. $\mathrm{NaCl}$ (0.05 M, $i=2$): $0.05\times 2=0.1$. $\mathrm{KCl}$ (0.1 M, $i=2$): $0.1\times 2=0.2$. The smallest effective concentration (0.1) belongs to 0.05 M $\mathrm{NaCl}$, so it has the minimum elevation in boiling point.