Question

A certain substance X has a normal boiling point of 134.5°C and a molal boiling point elevation constant =Kb1.36·°C·kgmol−1 . Calculate the boiling point of a solution made of 76.g of urea NH22CO dissolved in 850.g of X . Be sure your answer has the correct number of significant digits.

Answer 1

Answer:

136.5°C

Explanation:

Given data

• Boiling point of X: 134.5°C

• Molal boiling point elevation constant (Kb): 1.36·°C·kg.mol⁻¹

We can calculate the elevation in the boiling point (ΔT) using the following expression.

ΔT = Kb × m

where,

m is the molality

The molar mass of urea is 60.06 g/mol. The moles of urea corresponding to 76 g is:

76 g × (1 mol/60.06g) = 1.3 mol

The mass of the solvent (X) is 850 g = 0.850 kg.

The molality is:

m = 1.3 mol / 0.850 kg = 1.5 mol/kg

Then,

ΔT = Kb × m

ΔT = 1.36·°C·kg.mol⁻¹ × 1.5 mol/kg

ΔT = 2.0 °C

The boiling point of the solution is

134.5°C + 2.0°C = 136.5°C