Question

The molal freezing point constant for water is 1.86 K kg mol⁻¹. Therefore, the freezing point of 0.1 molal NaCl solution in water is expected to be ______.

Options

• −1.86°C

• −0.186°C

• −0.372°C

• +0.372°C

Answer 1

The molal freezing point constant for water is 1.86 K kg mol⁻¹. Therefore, the freezing point of 0.1 molal NaCl solution in water is expected to be −0.372°C.

Explanation:

Given: Molal freezing point depression constant, K_f = 1.86 K kg mol⁻¹

Molality of NaCl solution = 0.1 mol/kg

NaCl dissociates into 2 ions: Na⁺ and Cl⁻ i.e., i = 2

ΔT_f = i K_f m

= 2 × 1.86 × 0.1

= 0.372 K

Pure water freezes at 0°C.

Freezing point of solution = 0 − ΔT_f

= 0 − 0.372

= −0.372°C