If solute is volatile, upon heating to certain temperature it will also goes into vapour state. So, the pressure created by these vapour will the vapour pressure of pure solute.
When a liquid is pure and it boils , its vapour pressure will be equal to pressure exerted by pure liquid.
When a volatile solute is mixed with solvent then the total vapour pressure of solution is given by partial vapour pressure of solute and solvent. Here the partial vapour pressure is the individual contribution of vapour pressure of solute and solvent which is given by the Roult's law .
P1 = X1P10
P2 = X2 P20
Here P1 and P2 is the partial vapour pressure of solute and solvent individually in solution. P10 , P20 are the individual vapour pressure of solute and solvent in pure state.