The vapour pressure of a liquid depends on l. temperature. ll. rate of condensation. III. cohesive forces holding the particles together

Analyze the question: We need to decide which of the three factors determine the vapour pressure of a liquid.

Concept: Vapour pressure is the pressure exerted by a vapour in dynamic equilibrium with its liquid at a given temperature. It depends on how easily particles escape from the liquid surface into the gas phase.

Step-by-step evaluation:

• I. Temperature: Yes. Higher temperature gives molecules more kinetic energy, so more of them escape. Vapour pressure increases with temperature (this is why boiling happens when vapour pressure equals atmospheric pressure).

• II. Rate of condensation: No. At equilibrium, the rate of condensation equals the rate of evaporation. Condensation rate is a result of the vapour present, not an independent cause. It is not a determining factor.

• III. Cohesive forces holding the particles together: Yes. This is the strength of intermolecular forces. Weak forces (e.g. ether, ethanol) let molecules escape easily, giving high vapour pressure (volatile liquids). Strong forces (e.g. water, with hydrogen bonding, or mercury) give low vapour pressure.

So the vapour pressure depends on I and III only.

(Note: vapour pressure does not depend on the surface area or the amount of liquid, as long as some liquid is present.)

Tip: Remember that vapour pressure depends on two things: temperature and the nature of the liquid (intermolecular forces). Anything that is merely a consequence of equilibrium, like the condensation rate, or any physical quantity like volume or surface area, is not a cause.