A solution of a non-volatile solute in water freezes at . The vapour pressure of pure water at 298 K — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

$\Delta T_f = 1.0^\circ\text{C}$. Molality $m = \Delta T_f / K_f = 1.0 / 1.80 = 0.555 \text{ m}$. Mole fraction of solute $X_{\text{solute}} \approx m / (m + 55.55) \approx 0.555 / 56.105 \approx 0.0099 \approx 0.01$. Relative lowering of vapour pressure is $(P^\circ - P) / P^\circ = 0.01$. Thus, $P = P^\circ(1 - 0.01) = 24.24 \times 0.99 = 23.9976 \approx 24.00 \text{ mm Hg}$. Therefore, correct answer is A.