Question

Oxygen gas is collected by downward displacement of water in a jar. The level of water inside the jar is adjusted to the height of water outside the jar. When the adjustment is made, the pressure exerted by the oxygen is?
A. Equal to the atmospheric pressure
B. Equal to the vapour pressure of oxygen at that temperature
C. Equal to atmospheric pressure plus aqueous tension at that temperature
D. Equal to atmospheric pressure minus aqueous tension at that temperature

Answer 1

Hint: Oxygen gas is collected by downward displacement of \[{{\text{H}}_{\text{2}}}{\text{O}}\]. So, there will be some pressure exerted by \[{{\text{O}}_{\text{2}}}\]will be there and as the water level in test tube rising so aqueous tension also there, when water level outside = water level inside.

Complete answer:
Outside the jar, the pressure is equal to the sum of the atmospheric pressure and the aqueous tension.
When the level of water inside the jar is the same as that outside the jar, the pressure outside the jar is equal to the pressure inside the jar.
Thus, the pressure inside the jar is equal to the sum of the atmospheric pressure and the aqueous tension at that temperature.
But the pressure inside the jar is the sum of the pressure exerted by the oxygen and the aqueous tension at that temperature.

Hence, the pressure exerted by oxygen is equal to the atmospheric pressure minus the aqueous tension at that temperature.

Option D is the correct answer.

Note: Aqueous tension is defined as the partial pressure of the water vapour present in the moist gas. When the gas is collected over the water, it becomes moist due to the presence of small quantities of water vapour caused by evaporation.