Question

A solution is prepare using 45.2 g of an unknown (nonelectrolyte) dissolved in 476.4 g of water. The freezing point depression

of the solution compared to water is measured to be 1.87°C. What is the molar mass of the unknown (in g/mol)?
Some possibly useful constants for water are K= 1.86°C/m and Kb = 0.512°C/m.
Enter your value using three significant figures.

Answer 1

94.4g/mol is molar mass of the unknown

Step-by-step explanation:

Based on the freezing point depression equation:

ΔT = Kf*m*i

Where ΔT is the depression in freezing point (1.87°C)

Kf is freezing point depression constant of water (1.86°Ckg/mol)

And i is Van't Hoff factor (1 for nonelectrolyte solutes)

Replacing:

1.87°C = 1.86°CKg/mol*m*i

1.005mol/kg solvent = m

Using the mass of the solvent we can find the oles of the nonelectrolyte:

1.005mol/kg solvent * 0.4764kg = 0.479moles

Molar mass is defined as the ratio between mass of a substance in grams and moles, that is:

45.2g / 0.479mol =

94.4g/mol is molar mass of the unknown