Carbon dioxide concentration from pressure changes Is there a way of determining the concentration of CO2 if we know the pressure in the container before the addition of CO2 and the pressure in the container after the addition of CO2 if we have no way of measuring the amount of CO2 delivered to the container (like with a CO2 controller with a CO2 monitor)? For instance, if we have a sealed container with air in it at 298 K, whose pressure is measured at 1 atm, and we introduce carbon dioxide gas causing the pressure to increase to 1.2 atm... can we use the ideal gas equation and assume that the 0.2 atm increase is due to addition of CO2 and is the partial pressure of CO2 and just use 0.2 atm/(0.0821 × 298) = 0.008 m/L (approx. 352 ppm) as the concentration of the CO2 in the chamber? Or, is there more to it?
If you know the partial pressures of the gases, then you can calculate the mass fractions.  In the present case, assume you have 1 mole of gas.  Then the number of moles of CO2 is 0.167 moles and the number of moles of air is 0.833.  The mass of CO2 is (44)(0.167)=7.33 grams and the mass of air is (29)(0.833)=24.2 grams.  So the mass fraction of CO2 is $$\frac{7.33}{(7.33+24.2)}=0.233$$
So the ppm CO2 by volume = 167000 and the ppm CO2 by mass = 233000.