What is the total mass of 2.0 moles of h2 (

g.?

When we speak of hydrogen, the assumption is that it is hydrogen gas, H2, and not atomic hydrogen. Since hydrogen is diatomic, 1 mol of H2 has a mass of 2.0 grams. Therefore, 2 moles will have a mass of 4.0 grams.

Likewise, 0.5 grams is 1/4 of the mass of 1 mole (2.0 g), so the answer is 0.25 mol H2.

You can even set up a conversion factor to get the answer.

0.5 g H2 x (1 mol H2 / 2.0 g H2) = 0.25 mol H2