text: Argh...those other answers are way too complex...we don't need equations, save one: W = mg.\n\nNow...\n1. weigh the empty jar with air in it (W1)\n2. seal the air-filled jar\n3. pump out the air to form a vacuum inside\n4. weigh the vacuum filled jar (W0)\n5. note the difference in weight (W1 - W0) = W; the weight (not mass) of the air you pumped out\n6. 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.\n\nThere 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.