In order to solve this, it is recommended that you use stoichiometry. You need to use the mole hill and go from molecules to moles to grams. In order to go from molecules to moles, you must use Avogadro's number (6.02x10^23). Next, you need to go from moles to grams using atomic mass (I usually just go out to two decimal places). So, your atomic mass should be around 2.02g for H2. Your final answer should be around 3.1g H2 (don't forget significant figures!). Hope this helped