Because you are not seeing the obvious. He conc. is constant (assuming const. V).

So you don't need a non-trivial ODE for it.

Hmm - and Concentration=Partial Pressure/(RT), if the gases are ideal. So does this mean that the partial pressure of He also does not change, if the volume and temperature are constant, even as moles of the other gases change?

When put that way it seems simpler. The He present just does not affect our calculations. All other species' concentrations or partial pressures, we can calculate as before. If we want the total pressure, we can just calculate the partial pressure for all the reacting species, add them up and then add on He's partial pressure (which is the same as it was initially).

Even if volume is changing, should be ok: if we want to write volume as the sum of moles * molar volume of each species, we'd need to include He's number of moles, and then just find its molar volume (maybe a known function of the pressure and the temperature, maybe just a known value). Since He's number of moles is a constant, this will add as a constant to the volume (V

_{m}[He] will never be a function of n(He), only of overall pressure and temperature).

Correct?