An ideal solution has two component A and B. A is more volatile than B and also PA0 > Ptotal. If XA and YA are the mole fractions of A in the liquid and vapour phase then

XA, XB = MF of A & B in liq. phaseYA , YB = MF of A & B in vapour phaseA is more volatile than B and also        In vapour phase, according to Dalton's law      PA = Ptotal yA  Thus Exact no.of moles of A & B are not mentioned, exact relation between XA & YA can not be determined