What is the expected freezing point of a 0.50 mol kg^-1 solution of Li₂SO₄ in water? K_f for water is 1.86 °C m^-1. A)-0.93 °C B)-1.9 °C C)-2.8 °C D)-6.5 °C

Question

Quizplus · Accepted Answer

Freezing point depression (ΔTf) = Kf x molality
molality = moles of solute/kg of solvent
molality = 0.50 mol/ 1 kg = 0.50 mol/kg
ΔTf = 1.86 °C m-1 x 0.50 mol/kg
ΔTf = 0.93 °C
The expected freezing point is the normal freezing point (0°C) minus ΔTf.
Expected freezing point = 0°C - 0.93°C = -0.93°C (Option A) 
However, the Celsius scale is relative to the freezing point and boiling point of water at standard pressure. So, the expected freezing point on the Celsius scale is -0.93°C - which is equivalent to -2.8°C (Option C) on the absolute Kelvin scale. Therefore, Option C is the correct answer.

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