So as we go this way, the vapor pressure of compound B decreases. Now, at any given time, we could find a final vapor pressure and we find the final vapor pressure by summing the two pressures. So for example, say at this point the total pressure is this guy plus this guy gives you this plus this. So it's somewhere over here and that's what the red line is. Now this is for an ideal fluid. Now, in an ideal fluid the surface molecules are not connected by any intermolecular forces because remember, in an ideal fluid we're neglecting those intermolecular forces.