Question

why are gas jars full of hydrogen kept inverted?

Answer 1

Hydrogen gas, H 2 , is less dense than air, and so it rises. By keeping the mouth downwards, the hydrogen will remain in the test tube. The density of a gas is directly proportional to its molecular weight (see the Related Question about the density of oxygen and air to the left for why this is), and H 2 gas is much lighter than air. The average molecular weight of the molecules in air is 28.97 grams per mole, but the molecular weight of hydrogen gas is only 2.01 grams per mole. Hydrogen gas is more than 14 times less dense than air!