Knowing where to look it up - priceless
Carbon = 12 molecular wt
Oxygen = 16 molecular wt
CO = 28 molecular wt (?) +/- the odd sub-atomic particle
Air is about
78 % Nitrogen (14 molecular wt), actually N2 (= 28 molecular wt)
21 % O2 (=32 molecular wt)
1 % inert gases, water vapor (20 molecular wt), nitrous oxides and diesel particulates
I'd guess CO mixes pretty well with air. Might not matter too much about the detector height.
We installed our CO/Smoke detector at about shoulder level on the bulkhead near the entrance to the V-berth where we sleep. The installation directions didn't speak to an ideal height for the combo detector, IIRC.
PS - Wikipedia says:
CO detectors can be placed near the ceiling or near the floor because CO is very close to the same density as air.