Question

A solution containing 12.5 g of a non-electrolyte solute in 175 g of water gave boiling point of 100.70°C. Calculate the molecular mass of the solute.

(Given: K_b for water = 0.52 K kg mol⁻¹)

Answer 1

Given: Mass of solute = 12.5 g

Mass of solvent (water) = 175 g = 0.175 kg

Boiling point of solution = 100.70°C

Boiling point of pure water = 100°C

Molal boiling point constant (K_b) for water = 0.52 K kg mol⁻¹

First, we will calculate the boiling point elevation (ΔT_b),

T_b = Boiling point of solution − Boiling point of pure solvent

= 100.70 − 100.00

= 0.70 K

Now, we will calculate molality,

m = `"Moles of solute"/"Mass of solvent in kg"`

From the boiling point elevation formula:

m = `(DeltaT_b)/K_b`

= `0.70/0.52`

= 1.346 mol/kg

Now, we will calculate the moles of solute,

Moles of solute = m × Mass of solvent in kg

= 1.346 × 0.175

= 0.2355 moles

Now, the molecular mass of the solute will be,

Molecular mass = `"Mass of solute"/"Moles of solute"`

= `12.5/0.2355`

= 53.06 g/mol

Thus, the molecular mass of the solute is 53.06 g/mol.