Water is added to the solution M such that the mole fraction of water in the solution becomes 0.9. T — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

Now water is the solvent ($X_{\text{water}} = 0.9$) and ethanol is the non-volatile solute ($X_{\text{ethanol}} = 0.1$). Molality $m = X_{\text{ethanol}} / (X_{\text{water}} \times M_{\text{water}}) \times 1000 = 0.1 / (0.9 \times 18) \times 1000 = 100 / 16.2 = 6.17 \text{ m}$. Elevation $\Delta T_b = K_b^{\text{water}} \times m = 0.52 \times 6.17 = 3.21 \text{ K}$. Boiling point = $373 + 3.21 = 376.21 \text{ K} \approx 376.2 \text{ K}$. Therefore, correct answer is B.