Effect of an Inert Gas on Equilibrium

[brbrett]

Homework Statement

N₂O₄(g) + energy <---> 2NO₂(g)

How are Keq and [N₂O₄] affected by the addition of Ne, an inert gas, in a container at constant volume and temperature.
Keq [N₂O₄]
a) no change/no change
b) no change/ increases
c) increases/decreases
d) decreases/increases

Homework Equations

n/a

The Attempt at a Solution

I got an answer of A, however the correct answer is apparently B. If my understanding is correct, B is the correct answer because the pressure of the system has increased, therefore increasing the chemicals to a higher concentration. Equilibrium will be shifted left to attempt to restore the original ratio of concentration to volume.

Is this reasoning correct? I did some research on the internet, and a lot of people say the addition of an inert gas will not change concentrations if the volume is constant, just like this question. Also, Keq will not change regardless, as mathematically nothing has changed since the concentrations rise and fall in proportion with their molar ratios.
Thank you once again~

 

[Borek]

It depends on how you express concentration.

If as molarity - inert gas doesn't matter, "a" is the answer. Just take a look at the molarity definition - everything remains constant.

If as a molar fraction or a mass fraction (percentage) - then definitely concentrations go down. It doesn't matter for the equilibrium, as this is governed by partial pressures which remain constant.

No idea why the answer "a" is not accepted as a correct one though, if the question doesn't state what concentration type to use.