# Calculating Kp for gas mixture from picture

1. Mar 7, 2015

### CroSinus

1. The problem statement, all variables and given/known data
A chemical reaction is described by the equation N2O4 <> 2NO2. The pressure equilibrium constant for the given reaction is equal to 8 Pa. Which picture describes the equilibrium? (Please, see the attached file!).

2. Relevant equations
N2O4 <> 2NO2

3. The attempt at a solution
First I wrote the expression for the equilibrium constant of the given reaction. Kp = p^2(NO2) / p(N2O4). Then I tried to substitute partial pressures with the number of molecules for each type of the gass. It is not clear to me how can I use only number of molecules to replace partial pressures since I have the partial pressure squared in numerator. The pressure squared prevents for some things to be canceled in the expression for Kp if I apply Dalton's law for gas mixtures. Can somebody explain this to me, please.

Last edited: Mar 7, 2015
2. Mar 8, 2015

### Staff: Mentor

Agreed, there is not enough data to answer the problem.

For the first picture, assuming the total pressure is Pt, partial pressure of NO2 is $\frac 1 3 P_t$ and partial pressure of N2O4 is $\frac 2 3 P_t$. Then

$$K_p = \frac {P^2_{NO_2}}{P_{N_2O_4}} = \frac {(\frac 1 3 P_t)^2}{\frac 2 3 P_t} = \frac 1 6 P_t$$

Assume Pt=48 Pa and the first image is the correct answer. Problem is, the same can be done for every other image.

3. Mar 8, 2015

### CroSinus

Thanks for answering my question.
My textbook says the correct answer is C. But how did they get that result? Here's the original text of the question in Croatian.

Last edited: Mar 8, 2015
4. Mar 9, 2015

### Staff: Mentor

Making an error.

C is NOT the correct answer. There is no correct answer to this problem, as it is incomplete.

As I have shown you above - you can "prove" each answer to be the correct one, just assuming different values of the total pressure.

5. Mar 9, 2015

### CroSinus

Right. I agree with you. Unless you know the total pressure of the mixture, it is impossible to solve the problem. Funny textbook.