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 gas is:

• A. equal to the atmospheric pressure
• B. equal to the vapour pressure of oxygen at that temperature
• C. equal to the atmospheric pressure plus aqueous tension at that temperature
• D. equal to the atmospheric pressure minus aqueous tension at that temperature

Answer 1

The correct option is D equal to the atmospheric pressure minus aqueous tension at that temperature

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.