Question

A solution of urea (molecular mass = 60) contains 8.6 g per litre. It is isotonic with a 5% solution of a non-volatile and non-electrolytic solute. What will be the molecular mass of the solute?

Options

• 34.9 g mol⁻¹

• 349 g mol⁻¹

• 861 g mol⁻¹

• 3490 g mol⁻¹

Answer 1

349 g mol⁻¹

Explanation:

Given: Urea solution = 8.6 g/L

Molecular mass of urea = 60 g/mol

Therefore, Molarity of urea = `8.6/60` = 0.1433 mol/L

The solution is isotonic with a 5% (w/v) solution of another non-volatile, non-electrolyte solute.

Isotonic solutions have the same osmotic pressure.

Molarity of solute = Molarity of urea = 0.1433 mol/L

5% w/v means 5 g of solute in 100 mL, or 50 g in 1 L.

So, Moles of solute = `50/M`

∴ `50/M = 0.1433`

`M = 50/0.1433`

M = 349 g/mol⁻¹