pA and pB are the vapour pressure of pure liquid components A and B respectively of an ideal binary solution.If xA represents the mole fraction of component A, the total pressure of the solution will be

Total pressure,pT=pA′+pB′                    …………(i)We know that pA′=pAxA  and  pB′=pBxB                      pT=pAxA+pBxB            …………(ii)xA+xB=1⇒xA=1−xB or xB=1−xAFrom Eq. (ii)pT=pAxA+pB1−xA=pAxA+pB−pBxApT=pB+xApA−pB