# Calculating Partial Pressures, Keq

1. Feb 10, 2005

### relativitydude

We're doing chemical equillibrium and I think I pretty much have figured out how to set the table of initial, change and equilibrium pressures. However, i.e. how to arrange my Xs.

However, I do not understand some examples why some initial pressures are simply 0 ATM.

2. Feb 10, 2005

### dextercioby

The only reason why the initial partial pressure due to a chemical compound is zero is that,at the initial moment of time,it doesn't exist,so the partial pressure due to it has to be zero.I mean,if that compound were there from the start,wouldn't the pressure due to it be different from zero...?

Daniel.

P.S.I assume a similar analysis can be made for final states in a chem.reaction.

3. Feb 10, 2005

### relativitydude

Thank you, so, it is zero unless otherwise stated.

4. Feb 11, 2005

### Gokul43201

Staff Emeritus
No, there are only two reasons for something to have a zero initial partial pressure :

1. It is a product of the reaction, so at t=0, if X(A)=0, P(A) = 0
2. It is non-volatile, ie : its vapor pressure is negligible compared to the total pressure.

5. Feb 11, 2005

### dextercioby

Yes,Gokul is again,right.The second option didn't occur to me at all... :grumpy:

Daniel.