Hi, If you had a sealed round bottomed flask on some lab scales and you recorded the mass and then you removed all the air with a vacuum pump and then found the mass of the flask again would the difference between the two readings really be an estimate of the mass of the gas in the flask? The reason it puzzles me is that the body of the flask is clearly resting on the balance but the gas inside is randomly bouncing. OK so some of the gas particles will be hitting the bottom surface of the flask and hence pushing the flask down onto the scales a little bit ( but the force on the bottom then is to do with the rate of change of momentum of the bouncing air particles is it not? not their mass?) but equally gas particles will be hitting the top surface of the flask and lifting the flask off the scales a little bit. Can someone please explain to me why you can use this approach to estimate the mass of a gas. My discussion comes from an experimental set up suggested by the IOP.