Kelly E Jones wrote:
Salty Thumb wrote:
snip

or how adding atomic weights can give a correct indication of
density or buoyancy.

Because for most gases (at fixed temperature and pressure), the molar
volume (molar density) is roughly constant, thus the molecular weight
is a good indicator of the mass density, which determines bouyancy.

If we say that a mole of any gas occupies roughly 24 liters at STP,
and a mole of gas weighs it's molecular weight in grams, then the
density of any gas is proportional to it's molecular weight. The
density of CO2 is thus about 44 grams per 24 liters.

snip

This property can come in very handy, as long as you know a few
key atomic weights. Say, for instance, that you're stuck in a boring
class or lecture. No problem. Whip out your pencil and notepad and
calculate how much H2 it would take to float your neighbor's cat into
the stratosphere. Next, calculate how much He to do the same thing.
Look up at the lecturer from time to time, appearing thoughtful. He or
she will be impressed that you're taking more notes than any one else
in the room.

The above is a purely hypothetical scenario. g

R,
Tom Q.