# Homework Help: Help with calculating pressure at equilibrium (introductory thermodynamics problem)

1. Apr 9, 2010

### mandangalo

I am trying to calculate the pressure once a closed system reaches equilibrium at constant temperature and volume. I am to start with a tank filled with 1000L of at air 20oC and 1 atm. It is then filled with 900L propane at the same temperature without venting. Assuming the air doesn't dissolve into the liquid propane, what is the pressure at equilibrium.

I thought this was a case of figuring out what the partial pressure of the air is once the liquid propane has been added. However, once equilibrium is reached the total volume for the vapor phase is going to be greater than 100L. This is where my issue lies: How do I know how much of the propane is going into the vapor phase? Once I know how to calculate that I should be able to find out each partial pressure and sum them up to get the total pressure (unless my ideas behind that are wrong too).

Last edited: Apr 9, 2010