Question

The normal boiling point of ethyl acetate is 77.06 °C. A solution of 50 g of non-volatile solute in 150 g ethyl acetate boils at 82.60 °C. Calculate molar mass of solute. [Kb = 2.77°C kg mol⁻¹]

Answer 1

Given:

Boiling point elevation,

ΔTb = 82.60°C − 77.06°C = 5.54°C

Molality (m) =

`"ΔTb"/"Kb" = 5.54/2077 = "2mol"/"kg"`

Mass of solvent = 150 g = 0.150 kg

Let molar mass of solute = M

Molality = `"mass of solute (g)"/"M xx mass of solvent (kg)" ⇒ 2 = 50/(M xx 0.150)`

`2 = 50/(0.15 M) ⇒ "M" = 50/(2 xx 0.15) = 50/0.3` = 166.67kg mol

Molar mass = 166.67 g/mol