[LHM]

How is vapor pressure of a liquid in a closed container affected when the quantity of a liquid is doubled at constant temperature?

 A) The vapor pressure increases.
 B) The vapor pressure decreases.
 C) The vapor pressure stays the same.
 D) The vapor pressure may increase or decrease, depending on the liquid.

The answer is C. However, my logic was that if the quantity of liquid is doubled, then there is less space for the gas molecules to move around in, so the vapor pressure increases. Can someone tell me what's wrong with my logic?

Thanks

[eleven6eleven]

It is C because the vapor pressure is only dependent on temperature according to

ln VP = (-delta H)/(RT) + constant

This is called the Clausius-Clapeyron equation.

If there is less space for the gas molecules to move around in, then the molecules will simply revert back into liquid state until the vapor pressure is reached, i.e.

X(l) <--> X(g)

reaches equilibrium, where X is a molecule. So the pressure ends up staying the same, unless the temperature changes during the evaporation/condensation.