Ask an Explainer
Which has greater mass: air, water, or mercury?
That depends on whether you're trying to measure the mass of a molecule of each or if you're trying to measure a certain volume of each fluid. The molecular mass of air is about 28.96 grams per mole (or g/mol). The molecular mass of water is about 18.02 g/mol. And mercury is about 200.59 g/mol. So, a molecule of mercury would have the most mass.
But if you're talking about the mass of a volume of each, the real question is: which one is the most dense (has the most molecules in a given volume)?
To figure that out, you need to compare the same amount of air, water, and mercury. Let's use one cubic meter (about 35.31 cubic feet), because it's a standard measurement. One cubic meter of water weighs 1,000 kg (about 2,205 pounds). One cubic meter of air weighs 1.29 kg (about 2.85 pounds). And one cubic meter of mercury weighs 13,534 kg (about 29,837 pounds). In this example, although air has a higher molecular mass than water, a given volume of air has much less mass because there are fewer molecules of air in the given volume. Therefore, air is the least dense of the three fluids.
