Question

For a dilute solution, Raoult's law states that
A. The relative lowering of vapour pressure is proportional to the amount of solute in the solution
B. The relative lowering of vapour pressure is equal to the mole fraction of solute
C. The lowering of vapour pressure is equal to the mole fraction of the solute
D. The vapour pressure of the solution is equal to the mole fraction of solvent

Answer 1

Hint: Relative lowering of vapour pressure is a type of colligative property of the solutions. It is associated with the number and the amount of the solute entities in the solution.

Complete answer:
According to Raoult’s law, the value of the partial pressure of a component of liquid mixture is given by the multiplication of the vapour pressure of the component in pure form by the mole fraction of the component in that mixture.
This also envisages that the vapour pressure of the constituent component in pure state is always more than the vapour pressure of the solution at any specific temperature.
If the solute in the solution is volatile the vapour pressure of the solution will increase and if the solute is non volatile it will not contribute to the total vapour pressure of the solution and as a result the vapour pressure of the solution will decrease. The mole fraction of the solute is equal to the relative lowering of the vapour pressure.
Mole fraction of the solute is calculated by dividing the number of moles of solute by the total number of moles of the solution which comprises the number of moles of solute and the solvent. This relative lowering of vapour pressure is taken for a non-volatile solution.
Relative lowering of vapour pressure is concerned with the concentration of the number of moles of the solute in the solution. Being a colligative property it does not take into context the type of the solute particles
The correct answer is (B).

Note: Lowering of vapour pressure is the property of solutions in which the vapour pressure of the solvent is more than that of the solution. The boiling point of the solution increases due to decrease in the vapour pressure.