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?