1. The problem statement, all variables and given/known data A 10cm^3 bottle was filled with 10cm3 of air at atm. Later, 50cm3 more of air at atm was pumped in. What is the new pressure taking atm to be 10^5 Pa and temperature is constant? Their working was P1V1=P2V2 so 10^5 Pa .60cm^3 =P2. 10cm^3 and they solved P2 from there. But from this video, http://www.youtube.com/watch?feature=player_detailpage&v=GwoX_BemwHs#t=243s They said we cannot just apply that formula since n changes? 2. Relevant equations P1V1/T1=P2V2/T2 3. The attempt at a solution I think their working such that they already compile the gases together such that n is a constant as they added the initial 10cm^3 to the extra 50cm^3 when the gas was sort of in the atmosphere then when all of that was condensed into 10cm^3 the number of moles of gas remains constant so the equation is still correct. However, since there is 10cm^3 of atm air initially inside, is it safe to simply add them up as they are at atmospheric pressure so it's like assuming that the 60cm^3 of air is outside the bottle at first then later the entire bunch is condensed together to 10cm^3 later?