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?
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,
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:
W1 = mass of solvent
W2 = mass of solute
M1 = molecular mass of solvent
M2 = molecular mass of solute
M2 = 41.3469 g
∴ M2 ≈ 41.35 g
Therefore the molar mass of the solute is 41.35 g.
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