Question

Calculate the molar mass of a compound when 6.3 g of it is dissolved in 27 g of chloroform to form a solution that has a boiling point of 68.04°C. The boiling point of pure chloroform is 61.04°C and K_b for chloroform is 3.63°C kg mol ⁻¹.

Answer 1

Mass of solute = 6.3 g

Mass of chloroform (solvent) = 27 g or 0.027 kg

The boiling point of the solution = 68.04°C

The boiling point of the chloroform (solvent) = 61.04°C

ΔT_b = 68.04°C − 61.04°C

= 7.0°C

ΔK_b = 3.63°C mol⁻¹ kg

Molality, `m = (n_(solute))/("Solvent in kg")`

= `((6.3 g)/M)/(0.027 kg)`

ΔT_b = K_b.m

`7.0^\circC = 3.63^\circC  mol^(−1) kg^(−1) xx (6.3 g)/(0.027 kg xx M)`

`M = (6.3 g xx 3.63^\circC  mol^(−1) kg^(−1))/(0.027 kg xx 7.0 ^\circC)`

= 121 g/mol