When 1.47 g of an unknown non-electrolyte is dissolved in 50.0 g of carbon tetrachloride, the freezing point decreased by 7.03 degrees C. If the Kfp of the solvent is 29.8 K/m, calculate the molar mass of the unknown solute.

124.6 g/mol will be the molar mass for the unknown non-electrolyte

Explanation:

Freezing point depression →  ΔT = Kf . m

ΔT indicates the temperature variation:

Freezing point of pure solvent - Freezing point of solution

Kf is the cryoscopic constant → 29.8 K/m

Generally the unit is °C/m but we have to type the ΔT by K so let's replace the data given → 7.03 K = 29.8 K/m . m

m = 7.03 K / 29.8 m/K = 0.236 mol/kg

m means molality, the moles of solute in 1 kg of solvent. If we want to determine the moles of solute we do this operation:

molality . kg, so we may convert the mass of solvent from g to kg.

50 g . 1kg/1000g = 0.05 kg

0.236 mol/kg . 0.05 kg = 0.0118 moles

Molar mass → g/mol → So 1.47 g / 0.0118 mol = 124.6 g/mol

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