Question

What is the expected freezing point depression (°C) for a solution that contains 0.2 mol of magnesium sulfate dissolved in 1.0 kg of water? (Kf of water is -1.86 °C / m and normal freezing point is 0.0°C)

Answer 1

MgSO₄(s) --> Mg²⁺(aq) + SO₄^2-(aq).

1 mol of MgSO₄ produces 2 moles of particles when dissolved in water. Molality is equal to the moles of particles per 1 kg of the solvent. Therefore,

ΔT = i*m*K_f = 2 x 0.2 x (-1.86) = - 0.74^oC

Answer: -0.74^oC