# Vapour pressure and volume of container

Assume that a liquid is in equilibrium with its vapour in a container which can change volume. If we increase volume LE Chatelier principle states that pressure should increase but final pressure cant be equal to the initial pressure so final pressure will be less than initial pressure?

• Volume Increases => Pressure Increases. Le Chatilier's Principle does not say this. In fact it should be the opposite to this(even that is incorrect). Please elaborate your question. Jan 6 '19 at 15:09
• The question is ambiguous. Is the increase in volume carried out under isothermal conditions, adiabatic conditions, or some other conditions? Feb 2 at 20:21

Assuming you have a closed container to begin with, and you increase the volume via some mechanism to have more number of particles in the liquid, then you effectively are reducing the vapor pressure. Because the particles that were in the vapor phase have been forced to come down to the liquid.

• so why textbooks refer that vapour pressure depends only in temperature? Jan 8 '19 at 0:19
• You said that this is a closed container. If this container's lid was acting like a piston or something and that was what you used to change the volume, naturally the gas molecules would be compressed (but they don't want to get compressed - so they escape into the solution - because the interface between the liquid and the gas molecules would compose of strong intermolecular forces to drag those molecules back into the liquid). Jan 8 '19 at 8:01

From what I gather, it seems you are asking confirmation for the question you have posed, which is "Is the final pressure of a system, when its pressure is decreased, less than the pressure of the initial system?". If this is not the case, then your question is unclear.

Let's get the assumptions clear.

1. Le Chatelier's principle states that a system, when one of it's properties is changed, will act in such a way as to counteract this change.
2. The volume of a hypothetical system ($$A_{(l)}⇆A_{(g)}$$ for simplicity) is increased.

Increasing the volume of the system does not change the volume of the system in liquid state. It does, however, confer a decrease in the pressure of the gas, per Boyle's Law ($$P \varpropto \frac{1}{V}$$, given constant temperature). Le Chatelier's principle predicts that our hypothetical system counteracts this change by increasing the pressure of the gas to a final pressure. But this final pressure, as you have correctly claimed, is less than the initial pressure. As much as the system would like to return to the initial pressure, it cannot, because the ratio of gas particles to liquid particles must remain constant.

A decrease in the general pressure of the gas must also mean a decrease in the vapour pressure of the system. An increase in volume means that less gas particles per unit area are pushing on the surface of the liquid, which means that it is easier for liquid particles to escape into the gaseous phase. As the system stabilizes into equilibrium, the final vapour pressure would be lower than the initial vapour pressure.