Question

Freezing point of water is 0°C. At which temperature will one molal solution of NaCl freeze, if NaCl is taken to be completely dissociated (molal depression constant of water = 18.6)?

Options

• −3.72°C

• −1.86°C

• 0°C

• 3.72°C

Answer 1

−3.72°C

Explanation:

Given: Molarity (m) = 1 mol/kg

K_f = 1.86

NaCl dissociates completely into 2 ions, i.e., Van’t Hoff factor i = 2

∴ ΔT_f = i K_f m

= 2 × 1.86 × 1

= 3.72°C

Freezing point of pure water = 0°C

So, new freezing point = 0 − 3.72°C = −3.72°C