NEET 2020 Chemistry Question 105 Solution

Depression in freezing point ($$\Delta T_f$$) is a colligative property directly proportional to the molal concentration of the solute in the dilute solution: $$\Delta T_f = i \cdot K_f \cdot m$$

Because the solute is explicitly specified as a non-electrolyte, it undergoes neither association nor dissociation in benzene, so $$i = 1$$. - Cryoscopic constant ($$K_f$$) = $$5.12\text{ K kg mol}^{-1}$$ - Molality ($$m$$) = $$0.078\text{ m} = 0.078\text{ mol kg}^{-1}$$.

$$\Delta T_f = 5.12 \times 0.078$$ $$\Delta T_f = 0.39936\text{ K}$$

Rounding $$0.39936\text{ K}$$ to two decimal places gives: $$\Delta T_f \approx 0.40\text{ K}$$ This precisely matches Option (3).