Question

Question #b180a

Answer 1

Because hydrogen molecules and hydrogen atoms aren't the same thing.

Explanation:

Usually, when we talk about hydrogen atoms, we're referring to `H`; one mole, or `6.022 * 10^23` atoms of `H` is `"1.008 g"`.

When we talk about hydrogen molecules, on the other hand, we're usually referring to hydrogen as it's found in nature—diatomic hydrogen, or `H_2`.
The mass of one mole, or `6.022*10^23` molecules of `H_2` is `"1.008 g" * 2 = "2.016 g"`