Question

C6H6 freezes at 5.5°C. The temperature at which a solution of 10 g of C4H10 in 200 g of C6H6 freeze is ________ °C. (The molal freezing point depression constant of C6H6 is 5.12°C/m.)

Hint:

Freezing point {depression} means the solution always freezes at a {lower} temperature than the pure solvent.

Answer 1

Correct Answer: 1

Step 1: $\Delta T_f = K_f \times m$. Molar mass of $C_4H_{10} = 58 \text{ g/mol}$.
Step 2: Molality $m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}} = \frac{10/58}{0.2} = \frac{0.1724}{0.2} = 0.862 \text{ m}$.
Step 3: $\Delta T_f = 5.12 \times 0.862 \approx 4.41 ^\circ\text{C}$.
Step 4: $T_{freeze} = T^\circ_f - \Delta T_f = 5.5 - 4.41 = 1.09 ^\circ\text{C} \approx 1.1 ^\circ\text{C}$.