# An aqueous solution of 2% non-volatile solute exerts a pressure of 1.004 bar at the normal boiling point of the solvent. What is the molar mass of the solute?

Given:

2% non-volatile solute in an aqueous solution means that if the solution is of mass 100 g then the mass of solute in the solution is,

Mass of solute = [2 × 100]/100

= 2 g

Mass of Solvent = 100-2

= 98 g

Vapour pressure of solution at normal boiling point, P1 = 1.004 bar

Vapour pressure of pure water at normal boiling point, P1° = 1 atm = 1.013 bar

To find: Molar mass of solute

Formula:

From Raoult’s Law, we have, ………….. 

Now we know that number of moles of solute is given by the following relationship,

Number of moles Using the above relationship the equation  can be modified as follows: Where

W1 = mass of solvent

W2 = mass of solute

M1 = molecular mass of solvent

M2 = molecular mass of solute

Solution:  M2 = M2 = 41.3469 g

M2 ≈ 41.35 g

Therefore the molar mass of the solute is 41.35 g.

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