Question

10g of a non-volatile and non-dissociating solute is dissolved in 200g of benzene.

Answer 1

Answer: The molar mass of solute is 115 g/mol.

Explanation:

Elevation in the boiling point is defined as the difference between the boiling point of the solution and the boiling point of the pure solvent.

The expression for the calculation of elevation in boiling point is:

$\text{Boiling point of solution}-\text{boiling point of pure solvent}=i\times K_b\times m$

OR

$\text{Boiling point of solution}-\text{Boiling point of pure solvent}=i\times K_f\times \frac{m_{solute}\times 1000}{M_{solute}\times w_{solvent}\text{(in g)}}$ ......(1)

where,

Boiling point of pure solvent (benzene) = $80.10^oC$

Boiling point of solution = $81.20^oC$

i = Vant Hoff factor = 1 (for non-electrolytes)

$K_b$ = Boiling point elevation constant = $2.53^oC/m$

$m_{solute}$ = Given mass of solute = 10 g

$M_{solute}$ = Molar mass of solute = ? g/mol

$w_{solvent}$ = Mass of solvent = 200 g

Putting values in equation 1, we get:

$81.20-80.10=1\times 2.53\times \frac{10\times 1000}{M_{solute}\times 200}\\\\M_{solute}=\frac{1\times 2.53\times 10\times 1000}{1.1\times 200}\\\\M_{solute}=115g/mol$

Hence, the molar mass of solute is 115 g/mol.