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^{25}molecules per cubic meter while air at 1 Torr contains about 3.5 x 10

^{ 22}molecules per cubic meter. Again the distance (mean free path) between air molecules at atmospheric pressure is about 6.7 x 10

^{- 6}cms and at 1 Torr it is 5 x 10

^{-3 }cms . Therefore there will be enough molecules to pump out even when the pressure reaches 1 Torr. , since air expands to fill available space and will exert an equal pressure on all points within the container. Consider that at each stroke of the pump it has to shift a maximum of only 7.5 gms of weight (i.e. weight of 6 litres of air) atmospheric pressure is sealed off so it doesn’t count. Such a pump should need to lift only the weight of the air and its own weight (weight of piston connecting rod etc.) My question is how much power will it take to run. A minimum of about 250 Watts I am guessing. Any comments ?