1 Answer

1 mole of NaCl contains 2 moles of ions (1 mole of Na⁺ ions and 1 moles of Cl⁻ ions).
Molality of ions of the solution, m = [(0.50 mol) / (1.0 kg)] × 2 = 1.0 mol/kg
Cryoscopic constant of water, K = 1.85 °C•kg/mol
Freezing point depression = K m = (1.85 °C•kg/mol) × (1.0 mol/kg) = 1.85°C
Normal freezing point of water = 0°C
Hence, Freezing point of the solution = (0 – 1.85)°C = 1.85°C