Solutions

For an ideal solution, the total vapour pressure is given by Raoult's Law: where and are the partial vapour pressures of components X and Y, respectively. The partial vapour pressure of a component in a solution is given by: where and are the mole fractions of X and Y, and and are the vapour pressures of pure X and Y. Given:
The vapour pressure of pure X, mm Hg.
The vapour pressure of pure Y, mm Hg.
The mole ratio of X and Y is 3:2,
so:
Now, calculate the partial pressures: Thus, the total vapour pressure is:

The correct option is (D) :

Understand Weight by Weight Percentage (% w/w):
A 20% w/w solution means that the mass of the solute (urea) constitutes 20% of the total mass of the solution (urea + water). The formula for weight by weight percentage is: Where, Total Mass of Solution = Mass of Solute + Mass of Solvent. We need to check each option to see if it yields a 20% w/w solution.

Check Option (A): 6g urea dissolved in 24g water

Mass of Solute (urea) = 6 g
Mass of Solvent (water) = 24 g
Total Mass of Solution = 6 g + 24 g = 30 g
This solution conforms to the required composition.

Check Option (B): 20g urea dissolved in 80g water

Mass of Solute (urea) = 20 g
Mass of Solvent (water) = 80 g
Total Mass of Solution = 20 g + 80 g = 100 g
This solution conforms to the required composition.

Check Option (C): 10g urea dissolved in 40g water

Mass of Solute (urea) = 10 g
Mass of Solvent (water) = 40 g
Total Mass of Solution = 10 g + 40 g = 50 g
This solution conforms to the required composition.

Check Option (D): 4g urea dissolved in 16g water

Mass of Solute (urea) = 4 g
Mass of Solvent (water) = 16 g
Total Mass of Solution = 4 g + 16 g = 20 g
This solution conforms to the required composition.

Check Option (E): 15g urea dissolved in 30g water

Mass of Solute (urea) = 15 g
Mass of Solvent (water) = 30 g
Total Mass of Solution = 15 g + 30 g = 45 g
This solution does not conform to the required 20% w/w composition.

Options (A), (B), (C), and (D) all represent a 20% w/w solution of urea in water. Option (E) represents a 33.33% w/w solution.

Therefore, the solution that does not conform to the required composition is (E).