text: how do you measure the mass of air inside a bell jar?   argh...those other answers are way too complex...we don't need equations, save one: w = mg.  now... 1. weigh the empty jar with air in it (w1) 2. seal the air-filled jar 3. pump out the air to form a vacuum inside 4. weigh the vacuum filled jar (w0) 5. note the difference in weight (w1 - w0) = w; the weight (not mass) of the air you pumped out 6. calculate the mass = (w1 - w0)/g = w/g; where w1 is the air filled jar weight in pounds, w0 is the vacuum filled jar weight in lb, and g = 32.2 ft/sec^2.  the mass (m) unit is in slugs.  there will be some error because one cannot form a perfect vacuum.  but the answer you get (m = w/g) will be close enough for most purposes.