Question

A solution of 12.5 g urea in 170 g of water gave a boiling point elevation of 0.63 K. Calculate the molar mass of urea taking K_b = 0.52 K/m.

Answer 1

Given: Mass of urea (solute) (w₂) = 12.5 g

Mass of water (solvent) (w₁) = 170 g = 0.170 kg

Elevation in boiling point (ΔT_b) = 0.63 K

Molal elevation constant (K_b) = 0.52 K kg/mol

Molar mass of urea (M₂) = ?

`Delta T_b = K_b * m`

⇒ `Delta T_b = K_b * w_2/(M_2 * w_1)`

Rearranging the above formula

`M_2 = (w_2 * K_b)/(Delta T_b * w_1)`

= `(12.5 xx 0.52)/(0.63 xx 0.170)`

= `6.5/0.1071`

M₂ = 60.7 g/mol

∴ The molar mass of urea is 60.7 g/mol.