Define aqueous tension. Why is it subtracted from the total pressure to determine the pressure of a dry gas?

Ans: Aqueous tension is defined as the partial pressure of the water vapour present in the moist gas.
When the gas is colleced over the water, it becomes moist due to presence of small quantity of water vapour caused by evaporation.
Then, pressure exerted by moist gas = pressure exerted by dry gas + pressure of water vapour (aqueous tension).
i.e. Pmoist = Pdry + f (aqueous tension)
or, Pdry = Pmoist – f
Therefore, aqueous tension should be subtracted from the total pressure to get pressure of the dry gas.