When a gas is placed in an evacuated container,at a constant pressure ,then some of the liquid molecules pass into gaseous state and some of them condenses back. At dynamic equilibrium the vapor above the liquid is saturated and is called saturated vapor pressure.


It is said that presence of a gas such as air above the liquid in the container would decrease the rate of evaporation but it would not effect the equilibrium pressure of vapor.


BUT how can presence of air does not effect the equilibrium pressure of vapor?? :confused:


In fact the saturated vapor pressure of the liquid should decrease in presence of air molecules because the air molecules will certainly occupy some space and would decrease the number of liquid molecules and occupy little space relatively when in absence of air molecules therefore will have decreased vapor pressure.

When a liquid is placed into an evacuated container, some of the liquid molecules pass into gaseous state and some of them condense back to the liquid state. At dynamic equilibrium the vapor above the liquid is saturated and is called saturated vapor pressure.


This is correct as it is written, and corrected by me. Note that when you have an "evacuated container", there is nothing in it. The words "at constant pressure" don't mean much. Zero pressure might make more sense. After introducing the liquid, pressure inside the vessel is caused only by the vapor that comes evaporates from the liquid.



It is said that presence of a gas such as air above the liquid in the container would decrease the rate of evaporation but it would not affect the equilibrium pressure of vapor.


This is true. Air molecules collide with evaporating molecules from the liquid. This slows the rate of evaporation, but it doesn't affect the number of molecules that eventually evaporate and re-condense. The presence of air only affects the number of molecules in the space and thus the total pressure in the vessel.



BUT how can presence of air not affect the equilibrium pressure of vapor???

In fact the saturated vapor pressure of the liquid should decrease in presence of air molecules because the air molecules will certainly occupy some space and would decrease the number of liquid molecules and occupy little space relatively when in absence of air molecules therefore will have decreased vapor pressure.

No. To cause the saturated vapor pressure to decrease, a force must be applied to cause some of the substance molecules to condense. Air molecules cannot do that. All they can do is collide with the substance molecules and that doesn't do much. Therefore the vapor pressure doesn't change even though the *total pressure* does change.

In actuality, the total space occupied by molecules in the gas phase at ordinary pressures is pretty small. If the volume increased, more evaporation would take place to fill the space available, but the saturation vapor pressure of the substance would not change.

Thanks perito for answering my qestion patiently.your answer is very helpful indeed.I need a small clarification ,when a liquid in a evacuated container at costant temperature is saturated,then does saturation mean that entire space above the liquid is completely filled with gas molecules of liquid and cannot accommodate any further molecules of the gas?

You are confusing a macroscopic phenomenon (gas pressure) with a microscopic phenomenon (a molecule leaving liquid surface to become a vapor molecule). The presence of relatively small amount of air molecules does not change the rate with which liquid molecules leave the surface.

When a liquid in a evacuated container at constant temperature is saturated, then does saturation mean that entire space above the liquid is completely filled with gas molecules of the liquid and cannot accommodate any further molecules of the gas?

Yes. If more molecules of the liquid evaporate, the same number of molecules condense so the space continues to contain the same number of molecules.