Question

The molal elevation constant for water is 0.56°C per kg of water. Calculate the boiling point of solution made by dissolving 6 g of urea in 200 g of water.

Answer 1

Given: Molal elevation constant (K_b) = 0.56°C kg/mol

Mass of urea (solute) (w₂) = 6 g

Molar mass of urea (M) = 60 g/mol

Mass of water (solvent) (w₁) = 200 g = 0.2 kg

Boiling point of pure water `(T_b^circ)` = 100°C

Molality (m) = `w_2/(M xx w_1)`

= `6/(60 xx 0.2)`

= `6/12` 

= 0.5 mol/kg

ΔT_b = K_b × m

= 0.56 × 0.5

= 0.28°C

`T_b = T_b^circ + Delta T_b`

= 100 + 0.28

= 100.28°C

∴ The boiling point of the solution is 100.28°C.