By Michael Dempsey

Heres an interesting thought:
Imagine you are sitting in an small to average sized bar or a spacious lounge. Lets say its dimensions are 10M long x 3M high x 6M wide. If you could compress all the air in the room down to say, something like a shoe-box, could you pick it up and walk out of the room with it? Yes?…

For simplicity, let us say that the air is made up of 80 % Nitrogen and 20% Oxygen (not a bad approximation). The volume of the room is 10 x 3 x 6 = 180 M^3.

This is equal to 1000 x 180 = 180,000 dM^3 or 180,000 cubic litres. Now, non-chemists, physicists or engineers will have to take my word on this part:

1g mol of Nitrogen in typical air weighs 28 x 80/100 = 22.4 g and 1g mol of Oxygen in typical air weighs 32 x 20/100 = 6.4g.

So, (22.4 + 6.4) = 28.8g.

That is: 1 mol of air molecules weigh 28.8g and occupy 22.4 dM^3 at 1 atmosphere pressure and 0 degrees C. (OK, its a chilly room!).

So, the actual volume of the room is 180,000 dM^3, therefore the mass of the air in the room is 180,000/22.4 x 28.8 = 231429g or 231 Kg!

I dont think many of us could lift a shoe-box weighing 231 Kg even if it had handles on.

So, our shoe-box (of negligeble weight) measuring 200mm x 120mm x 350mm containing the compressed air weighs 231 Kg.

The same shoe-box, if it contained pure lead, would weigh: (specific gravity of lead)11.34 x 20 x 12 x 35 = 95256g or 95 Kg.

In other words, the shoe-box filled with compressed air would be about two-and-a-half times heavier than the one full of lead!