Question

A certain substance X condenses at a temperature of 123.3°C . But if a 650. g sample of X's prepared with 24.6 g of urea ((NH2)2 CO) dissolved in it, the sample is found to have a condensation point of 124.3°C instead. Calculate the molal boiling point elevation constant Kb of X.

Answer 1

1.6 °C.kg/mol

Step-by-step explanation:

Step 1: Calculate the molality of urea

We will use the following expression.

m = mass(urea) / molar mass(urea) × kg solvent

m = 24.6 g / 60.06 g/mol × 0.650 kg

m = 0.630 mol/kg

Step 2: Calculate the boiling point elevation of X

The boiling point elevation is a colligative property that can be calculated using the following expression.

ΔTb = 124.3 °C - 123.3 °C = 1.0 °C

Step 3: Calculate the boiling point elevation constant

We will use the following expression.

ΔTb = Kb × m

Kb = ΔTb/m

Kb = 1.0 °C/(0.630 mol/kg) = 1.6 °C.kg/mol