The molar mass of substance which forms 7.0% (by mass) solution in water which freezes at . The cryo — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

A 7.0% (by mass) solution implies 7.0 g of solute in 93 g of water. $\Delta T_f = 0.93^\circ\text{C}$. $\Delta T_f = K_f \times (w / (M \times W_{\text{kg}})) \Rightarrow 0.93 = 1.86 \times (7.0 / (M \times 0.093))$. $0.93 = 1.86 \times (75.268 / M) \Rightarrow M = (1.86 \times 7.0) / (0.93 \times 0.093) = 13.02 / 0.08649 = 150.53 \text{ g mol}^{-1}$. Therefore, correct answer is B.