I've lately been very interested in gasses and how they change as they move between containers, especially mixtures of gasses. I have an idea that I think would make a neat game, since that's a little hobby I have. The problem is, I know so little about the subject, I don't know what I don't know. So, the question is this: Suppose I have a tank of a certain size that is holding a known mass of one gas, and I know what the temperature and pressure is inside the tank. Then, if I add some certain mass of a different gas, that has some known temperature, into the tank, what will the new pressure inside the tank be?

I'm sure there's a formula for this, but I never went to college. Any help would really be appreciated!