What is the total mass of 2.0 moles of h2(

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