Effect of inert gas on Q Adding an inert gas will not affect Q if that reaction is held at a constant volume. But, if the reaction is held at constant pressure, then adding inert gas will change Q bc the partial pressures of the gases will change.

I don't understand this. If the reaction is held at constant pressure, this means that you changed the volume when adding the inert gas. Wouldn't the partial pressures of all the reactants change by the same factor, so Q wouldn't change?
Wouldn't the partial pressures of all the reactants change by the same factor, so Q wouldn't change?


If the number of gas particles is the same for reactants and products, then Q would not change. However, if you have a reaction like 

$$\ce{2N2O(g) &lt;=&gt; N2O4(g)}$$

even though the reactant and product partial pressure changes by the same factor, the changes don't cancel out because the coefficients are different.

$$ Q = \frac{[\ce{N2O4}]}{[\ce{N2O}]^2}$$