Question

Pure CS2 melts at -112°C. 228 grams of propylene glycol crystals is mixed with 500 grams of CS2. If kf of CS2 = -3.83 K kg mol^-1 what is the depression in freezing point?

(a) -23°C

(b) -135°C

(c) -20°C

(d) -100°C

This question was posed to me in exam.

The origin of the question is Colligative Properties and Determination of Molar Mass topic in portion Solutions of Chemistry – Class 12

Answer 1

The correct option is (a) -23°C

The best I can explain: Given,

kf = -3.83 k kg mol^-1

Mass of solute, w2 = 228 g

Mass of solvent, w1 = 500 g

Molar mass of solute, M2 = 76 g/mole

Moles of solute = w2/M2 = 228/76 = 3 moles

Molality of the solution, m = Number of moles of solute/mass of solvent (kg)

m = 3 moles/0.5 kg = 6 molal

We know, ΔTf = kf x m

ΔTf = -3.83 x 6 = -23°C.