The gas collected over water is called moist and pressure exerted by saturated water vapour is called aqueous tension.

The total pressure of the gas is P _{moist gas} = P _{dry gas}

As exerted water vapour is present i.e. aqueous tension.Which is correction term from total pressure.

By applying this correction term we get a new equation,shown below -

P _{dry gas} =P _{moist gas} – aqueous tension

Therefore, the correction term applied to the total pressure of the gas to get the pressure of dry gas is P _{moist gas} – aqueous tension