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A sealed container holds air above liquid water. The total pressure is 800 torr and the aqueous tension (vapour pressure of water) is 40 torr. If the volume of the container is doubled at constant temperature, what is the final total pressure?

1. 760 torr
2. 420 torr
3. 400 torr
4. 380 torr

Correct answer: 420 torr

Solution

The total pressure is the sum of partial pressure of air (P_air) and vapour pressure of water (P_water = 40 torr). Initially P_air = 800 - 40 = 760 torr. Since the liquid water is still present after doubling the volume, the vapour pressure of water remains 40 torr (it is an equilibrium value at constant temperature). By Boyle's law, P_air doubles its volume so new P_air = 760/2 = 380 torr. Total pressure = 380 + 40 = 420 torr.

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