I had an exam question where I had to find the volume taken up by a gas consisting of 32 grams of oxygen and 4 grams of helium at STP conditions. I ended up getting a supposedly incorrect answer of about 44.8 liters by finding the volume of both gases and adding them together. However, my instructor said that it's actually half that, 22.4 liters, and that you don't add the volume of two gases together in this situation. When I asked why, he couldn't really explain it in any way that made sense to me and we got into a confusing discussion over when the ideal gas law applies and when it doesn't. The whole thing ended up confusing me. So, in this situation do you add the volume taken up by the two gases together or not?