Homework Help: Partial pressure vs mole fraction graph

1. May 5, 2015

mooncrater

1. The problem statement, all variables and given/known data
There is a statement in a book :
" Graph of P vs $\chi$ is a straight line which $cannot$ pass through origin"

2. Relevant equations

3. The attempt at a solution
But if mole fraction of a component is zero then it can't form vapours because of which its partial pressure will be zero. So the P vs $\chi$ Graph $should$ pAss through origin. So is this given line wrong?

2. May 6, 2015

Staff: Mentor

It looks completely wrong to me.

3. May 6, 2015

T Damen

What would a molar fraction of 0 mean anyways? It means that there is no amount of the constituent present in the mixture. At molar fraction zero the graph does not describe the component anymore but anything that is not present in the mixture. And I guess that isn't really logical..

4. May 6, 2015

Staff: Mentor

Molar fraction of zero means there is no substance present, that's perfectly logical to me.

5. May 6, 2015

T Damen

That is perfectly logical of course..
But imagine a P, xi plot for substance x in a mixture. At xi=0 the plot is not meaningful for describing substance x anymore cause substance x is not present anymore.
Simply put: a P, xi plot for a certain substance can only be plotted for values of xi where the substance is actually present.

6. May 6, 2015

Staff: Mentor

What you are saying is that you object to extending the relationship p = Px down to x = 0, even though at x = 0, the species is not present so that its partial pressure p is equal to zero ( p = 0). And somehow this objection makes sense to you (because it doesn't make sense to me). I'm totally in agreement with Borek.

Chet