Tf(water) - Tf(soln) = kf mwhere Tf(water) is the freezing point of water, Tf(soln) is the freezing point of the solution, kf is the apparent molal freezing point constant (1.86 °C kg/mol, for water), and m is the total molality of particles in solution. If you assume that the potassium sulfate dissociates completely in solution, m = 3· 0.1 = 0.3 mol/kg for your problem. That makes the freezing point depression equal to about (0.3 mol/kg)×(1.86 °C kg/mol) = 0.56°C, so the freezing point of the solution is roughly -0.6°C.
Why is this only a crude estimate? The relationship was derived by assuming a dilute, ideal solution. Your solution doesn't meet either of those requirements. The ions (being charged) interact rather strongly in solution, producing deviations from ideal behavior that are difficult to ignore. Also polyvalent ions like sulfate tend to form ion pairs (e. g. KSO4-(aq)) at concentrations this high, so the assumption of complete dissociation isn't correct.
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