Question

A cylinder fitted with a moveable piston contains liquid water in equilibrium with water vapour at 25. Which operation results in a decrease in the equilibrium vapour pressure?
(A) Moving the piston downward a short distance
(B) Removing a small mass of vapour
(C) Removing a small mass of liquid water
(D) Dissolving salt in water

Answer 1

When impurities are added to a solution it always results in lowering of the vapour pressure. In terms of liquid, vapour pressure is a colligative property which means it depends on the fraction of solute and a solvent.

Complete step by step solution:
If the pressure is applied on one side then the remaining portion of the pressure is transferred within the same amount and hence the pressure is inversely proportional to area. Vapour pressure also known as equilibrium vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phase at a given temperature in a system which is closed. When sodium is added to liquid water, the equilibrium vapour pressure will be lowered because there will be a rise in solute particles which will fill the free space between water. As more sodium is added , there will be less number of solvent particles on the surface and will transform into gas phase due to less number of solvent particles.
Therefore, resulting in lowering of vapour pressure when sodium is added.
Hence the answer is “Dissolving salt in water”, option (D).

Note:
Vapour pressure is a measure in terms of standard units of pressure. Pressure is a force per area and the PASCAL (Pa) as its standard unit. One pascal is one newton per square meter
$(N\cdot{m^{ - 2}}\;or{\text{ }}kg\cdot{m^{ - 1}}\cdot{s^{ - 2}}).$