Air is mostly (70%) Nitrogen, most of the rest is Oxygen.

The atomic mass of Nitrogen is 14, of Oxygen is 16. Both occur as
molecules (N2, O2) so the mass of each molecule is twice the above
figures.

Now, at STP, there are 22.4 liters in a mole, and a mole is the amount
of stuff that would weigh (in grams) what its molecular mass is. So,
22.4 liters of nitrogen would weigh 28 grams. Of Oxygen, it would be
32 grams. So we have a little over a gram per liter. Ok, more than
just a little, but less than a gram and a half per, and it's in the
ballpark. Lets use one gram per liter.

A liter is 1000 cubic centimeters, which is the volume of a cube 10
centimeters on a side. (or equivealently, 1/10 meter on a side).
1000 liters would be a cube one meter on a side, and air would weigh
"a little" over one Kg per cubic meter. A meter is "a little" over
three feet, so a cubic meter is "a little more" over 27 cubic feet.
One Kg is "a little" over two pounds, divide by 27, or even 30, and I
get something less than a tenth of a pound per cubic foot.

Air is pretty heavy, which is why we can fly. It weighs .078
lb/cubic foot at standard sea level pressure and temperature. What's
that, about 13 cubic feet for a pound?

Yep. That's just about right.

Now, take a typical room that's three meters tall, three meters wide,
and four meters deep. Not a very big room, but it has a high ceiling.
This gives us 36 cubic meters, and the air would weigh "a little"
over 36 Kg. Well, I weigh more than a little over 36 Kg, but it's
close enough to show that a smallish room will hold less than a
person's mass in air, but a largish roomfull of air can easily
outweigh a person.

The air in a room can easily
outweigh the occupants.

Yep again.

Now, how big is the White House?

Jose
--
Freedom. It seemed like a good idea at the time.
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