A gas occupies 100cm3 when collected over water at 15°C and 750 mm pressure. It occupies 92cm3 in dry state of S.T.P. Find the aqueous tension(mm) ( or vapour pressure of water) at 15°C.

Let aqueous tension at 15∘C=P;At N.T.P P1=(750−P)mm,V1=100cm3T1=15+273=288KP2=760mm,V2=92cm3T2=273K.But: P1V1T1=P2V2T2 (gas law)(750−P)×100288=760×92273750−P=760×92×288273×100=737.6mm∴P=750−737.6=12.4mm