Doug Miller wrote:
It is weight per unit of volume at the ambient temperature that is
important.

Well, yes, but a rule of thumb for gases is that density at standard
temperature and pressure is approximately proportional to molecular
weight.

Taking air to be a mixture of 78% N2 (molecular weight 28), 21% O2
(32), and 1% Ar (18), then CO, with a molecular weight of 28, would
be expected on that
basis to have a density 0.97 that of air.

The actual figure is 0.969 (1.25 g/l for CO vs 1.29 for air).

The problem with all this figuring and carry-the-two business is that air is
a mixture - a relatively constant mixture. If not, then the bottom 2.25 feet
of a room would be all Nitrogen, the next 2.5 feet Oxygen, and so on. We'd
have to walk around bent over (toddlers would have to walk on stilts) or we
would perish.