Question

Which of the liquids in each of the following pairs has a higher vapour pressure?
A.Alcohol, glycerine
B.Mercury, water
C.Petrol, Kerosene
D.None of these

Answer 1

Hint: The vapour pressure of liquid is the pressure exerted by gas above the liquid surface in a closed vessel at a given temperature. The vapour pressure of the liquid at equilibrium depends on the following factors i.e. intermolecular forces of attraction and temperature of the liquid. The vapour pressure of liquid does not depend upon volume of the liquid.

Complete answer:
We know that vapour pressure of the liquid at any given temperature is defined as the pressure exerted by the vapour above the liquid surface at equilibrium with liquid at that particular temperature.
The equilibrium condition occurs when the rate of evaporation is equivalent to the rate of condensation. This phase is called the equilibrium stage.
The vapour pressure depends on some factors such as:
Nature of liquid- This is based on the intermolecular forces of attraction. As the intermolecular forces of attraction increase, the vapour pressure decreases.

Temperature – This factor relates to the kinetic energy of the liquid molecules. When the temperature increases, the kinetic energy i.e. the escaping tendency of molecules increases. Hence, the vapour pressure also increases.

Concentration of the solute- Due to the presence of solute particles, there will be reduction in the vapour pressure. This decrease in vapour pressure varies with respect to the concentration of solute.
The vapour pressure is also independent of some factors such as surface area, volume of the solution and humidity.
In the given question, Alcohol and glycerine have higher vapour pressure than mercury, water and petrol, kerosene. This is because alcohol and glycerine have less intermolecular forces of interaction as compared to petrol, kerosene and mercury, water. Petrol, kerosene, mercury and water have strong forces of attraction, so they have lower vapour pressure.
And we know that vapour pressure depends on the intermolecular attractions. As when the intermolecular forces increase, vapour pressure also decreases and vice versa.
The correct answer is option (A).

Note:
Don’t get confused with the effect of temperature and humidity. Vapour pressure is dependent on temperature and independent on humidity. Also, vapour pressure of a liquid is low then its boiling point will be high. In other words, vapour pressure is inversely proportional to the boiling point of a liquid.