1. The problem statement, all variables and given/known data Calculate the pressure ratio of He to N2 at which helium would have the same density as nitrogen if their temperatures were the same. 2. Relevant equations I used D = m/v 3. The attempt at a solution DHe = mHe/v DN2 = mN2/v Both gases occupy the same volume, so just v for both. Since DHe = DN2, mHr/v = mN2/v and mHe = mN2 For some x and y, x mol He(4.003 g/mol He) = mHe y mol N2(28.01 g/mol N2) = mN2 4.003x g = 28.01y g x = 7y To me it looks like there are 7 times as many moles of He as N2 but I doubt that would directly apply to their pressure ratios. I think I'd have to use PV = nRT but I'm not sure how I'd put it in. I was actually helping some chemistry students earlier today with this and am hoping I can have the answer ready for them tomorrow morning.