# Addition of a Volatile Solute to a Volatile Solvent, Both of which are Immiscible

When a volatile liquid is added to another volatile liquid, which are completely immiscible with each other, each liquid will be independent of the other and will exert its own vapor pressure. The more dense liquid (A) will form the lower layer and the less dense liquid (B) will form the upper layer. The hot liquid (B) is passed into liquid (A).

The total vapor pressure of the system will be the sum of vapor pressures of both the volatile liquids. Therefore,

(as XA and XB = 1)

The total vapor pressure of the system remains constant as long as both the volatile liquids are present and will not depend on the relative amounts of the two liquids.

where and represent the mole fractions of A and B, respectively, in the vapor phase. Let nA and nB are the moles of A and B, respectively, in the vapor phase. Therefore,

âˆ´

Thus, the masses of the constituents in the vapor (distillate) will be proportional to their respective molar mass and vapor pressure.