Calculating Kp if you only have limited data....

OK, some of you may have read my previous OP on my equilibrium constant woes, and now I have another (but final) question.

Dont worry, I’m not asking you to do my homework, merely how I can go about doing it myself…

Right, I’ve been asked to work out Kp (and the mole fractions and partial pressures of the gasses, but you need them for finding Kp) for a system;
N[sub]2[/sub]O[sub]4(g)[/sub] <==> 2NO[sub]2(g)[/sub]

I’ve only been given limited data, and I can’t quite see how to use it:

  • The pressure of the system is 0.200atm
  • At equilibrium, the relative molecular mass of the equilibrium mixture was 59.8g mol[sup]-1[/sup]

How do I work it out with only these?

I know that once I have the mole fractions, it will become easy, because…

Kp = (P[sub]NO2[/sub])[sup]2[/sup] / (P[sub]N2O4[/sub])

Do I assume that I have a certain amount in total, even though it’s not given, and work out the mole fractions from there?

Sorry to bother you people again, but I was absent from college last week due to illness, so I missed my chance to get help from my teacher, and it’s due in next week.

Help please!

Thanks in advance,


You can calculate the molar fraction of each species given their molecular mass and the relative molecular mass of the system at equlibrium.

Does that work? I thought that the mole fraction was defined as the amount of gas, G / total number of moles

I thought about this, since they would be in the same ratio.

However, the mole fraction is to do with the amounts at equilibrium, and the the molar mass of the two gases has nothing to do with the equilibrium.

I’ll give it a try now and see if the right numbers come out.



OK, I’ve worked out how to do it now, and I did it on my own!

I’d been thinking about it this morning, and looking through my previous notes, I’d written down the relationship;

M[sub]r[/sub] = {sigma}(M[sub]g[/sub] x[sub]g[/sub])

and we have the relationship;

{sigma}x[sub]g[/sub] = 1

With these two, I put them together as simultaneous equations, and managed to get the mole fractions, and could therefore complete the question