Question: how many moles of hydrogen do u have?
Answer: In this case, the periodic table tells us there is 1g of hydrogen per mole. We know that dihydrogen (H2) has 2 hydrogen atoms per molecule, so there will be 2 * 1g hydrogen per mole = 2g dihydrogen per mole. If there are 2g dyhydrogen per mole, and we have 4g, then we know 4g / 2g per mole = 2 mole dihydrogen in 4g.