Question

A solution is prepared by dissolving 7.00 g of glycerin (c3h8o3) in 201 g of ethanol (c2h5oh). the freezing point of the solution is ________°c. the freezing point of pure ethanol is -114.6°c at 1 atm. the molal-freezing-point-depression constant (kf) for ethanol is 1.99°c/m. the molar masses of glycerin and of ethanol are 92.1 g/mol and 46.1 g/mol, respectively

Answer 1

1) Freezing point depression is a colligative property and it follows this equation:

ΔTf = Kf * m

2) Kf = 1.99 °C /m

3) m = moles of solute / Kg of solvent

moles of solute = mass in grams / molar mass

moles of C3H8O3 = 7.00 g / 92.1 g/mol = 0.076 mol

Kg of C2H5OH = 201 g * 1 kg / 1000 g = 0.201 Kg

=> m = 0.076 mol / 0.201 kg = 0.3781 m

4) ΔTf = Kf * m = 1.99°C/m * 0.3781 m = 0.75°C

Tf = -114.6°C - 0.75°C = -115.35°C

Answer: - 115.4 °C