What is mole fraction of components in vapour phase?

Composition of vapour in equilibrium with the solution can be determined by Dalton's law of partial pressures

If y1 and y2 are mole fractions of two components in the vapour,

then P1=y1P and P2= y2P. where P and P, are the partial pressures of two components in the vapour and P is the total pressure.