Q. 154.2( 29 Votes )

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?

Answer :


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


From Raoult’s Law, we have,

………….. [1]

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

Number of moles

Using the above relationship the equation [1] can be modified as follows:


W1 = mass of solvent

W2 = mass of solute

M1 = molecular mass of solvent

M2 = molecular mass of solute


M2 =

M2 = 41.3469 g

M2 ≈ 41.35 g

Therefore the molar mass of the solute is 41.35 g.

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