Freezing point depression is a colligative property.
The formula that rules it is:
ΔT f = Kf * m
Where m is the molality of the solution => m = moles of solute / Kg of solvent
=> m = 0.31 mol / 0.175 kg = 1.771 m
And kf = 1.86 °C/m
=> ΔTf = 1.771 m * 1.86 °C / m = 3.3 °C
=> Freezing point of the solution = normal freezing point of water - 3.3 °C = 0 - 3.3°C = - 3.3°C.
Answer: -3.3 °C
