So pretend we have two samples of pure benezene in equilibrium with its vapor. One sample containing 0.5 mol and one containing 1 mol. They're also both at the same temperature. Let's pretend they're both at, say, 298 K (room temperature). Would the two samples have the same vapor pressure? I was thinking since the 1 mol sample has more molecules that could be vaporized, it would produce a greater pressure. But I was told this is wrong.
The vapour pressure under the circumstances of fixed T and n (amount of substance) depends on an additional constraint: either the volume of the container (if rigid) or the value of an applied external pressure.
In a rigid container (controlled fixed volume), as the volume is increased more of the liquid vaporizes but the vapor pressure remains constant until all of the substance is in the gas state. Past that point the pressure will decrease with increasing volume according to Boyle's law.
Under controlled constant pressure there will be vapor present if the applied pressure is smaller that the equilibrium vapor pressure at that temperature. At the boiling point (when applied pressure is equal to the applied pressure) one can transiently alter the applied pressure to allow increased evaporation, thereby increasing the volume for instance, but retaining the same intial and final pressures, at least until all of the substance has evaporated.
All that changing the amount of substance does is change the extent to which the volume will change when the liquid is converted entirely to gas. Between that point and the point at which there is no gas (because applied pressure is higher than vapour pressure, or because available volume is too small, which amounts to the same thing) the vapour pressure will be constant.