Question

The molal elevation of an unknown solution is equal to the molal elevation of 0.1 M solution of urea. The concentration of unknown solution is ______.

Options

• 1 M

• 0.01 M

• 0.1 M

• none of these

Answer 1

The molal elevation of an unknown solution is equal to the molal elevation of 0.1 M solution of urea. The concentration of unknown solution is 0.1 M.

Explanation:

Given: Molal elevation (i.e., boiling point elevation) of an unknown solution is that of 0.1 M urea solution.

Urea is a non-electrolyte, so i = 1

Boiling point elevation is a colligative property, dependent on 

ΔT_b​ = i ⋅ K_b ⋅ m

The unknown solute has the same molal elevation, so

`i_"unknown​" * m_"unknown" ​= 1 xx 0.1 = 0.1`

If the unknown solute is non-electrolyte, then i = 1, so

`m_"unknown" = 0.1` mol/kg

So, the concentration of the unknown solution is 0.1 M (assuming dilute aqueous solution where molality ≈ molarity).