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

Let

*X*and_{A}*X*are the mole fractions of two volatile miscible liquids_{B}*A*and*B*having vapor pressures and , respectively, at temperature*T*kelvin. Therefore, vapor pressure of solution is given by*P*

_{Total}=

*P*+

_{A}*P*= + (1 â€“

_{B}*X*)

_{A}A plot between

*P*_{Total}and*X*is linear with slope equal to and intercept equal to Thus, the total pressure increases with increase in_{A}*X*._{A}# Graphical representation

If and are the mole fractions of

*A*and*B*in the vapor phase, then according to Daltonâ€™s law of partial pressure, we can writeâˆ´

and

Now, the vapors above the solution are collected and condensed. The condensed liquid is brought to the same temperature at which original liquid mixture was kept. Then, the vapor pressure (

*P**â€™*_{Total}) of the resulting solution is given bywhere and are partial vapor pressures of

*A*and*B*, respectively, in the resulting solution.Let and be the mole fractions of

*A*and*B*in the vapors formed above the condensed liquid kept at the same temperature. Therefore,