Problem 11.43

[E_Villavicencio 2N]

This problem says:
Consider the reaction 2NO2(g)<--->N2(g)+O(g). If the initial pressure of NO is 1.0 bar, and x is the equilibrium concentration of N2, what is the correct equilibrium relation?

The answer is K=x^2/(1.0-2x)^2

My question was: the problem gives the pressure of one and the concentration of the other. Aren't we supposed to convert x to pressure in order to work with the same magnitude?

[Tyler_Ash_3C]

Since volume is not clearly defined in this problem, I think that it is acceptable to assume that at whatever volume the reaction is taking place, the pressure of NO2=1.00 bar while the concentration of the products is the same as their partial pressures. There should exist some volume where this holds true, so we assume that the reaction is taking place at that unspecified volume so that the pressure vs. concentration problem can be overlooked for the sake of solving the problem.

[Crystal_Chung_1A]

Hi,
If they're asking for a K value, using the concentration in molarity for Kc and partial pressure for Kp are both the same since they're both ratios. So, it's not necessary to x to pressure.
Hope that makes sense!