No, I mean when you take the same quantity. A single unit of water is lighter than a single unit of air. H2O has a molecular weight of 18, air, to first approximation, a molecular weight of 29.
By "air" I mean the standard current atmosphere of Earth. This is about 20% O2 and 80% N2. Everything else is too trace for weight discussion. .2*32+.8*28 is approximately 29.
Ah. That explains the methodology but it's still not exactly true because weight is also a product of density. A liter of water is heavier than a liter of air whether or not the constituent molecules are heavier. We could just say "on a molecular level water weighs less than oxygen" for the fun fact
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