Using the periodic table, we can get the atomic mass, which tells you how many grams of hydrogen is in 1 mole. 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.

To convert between moles and number of molecules or atoms, you need to use Avogadro's number, which is 6.022²³, or 6 with 23 zeros. Avogadro's number tells you how many particles there are per mole (there are 6.022²³ particles in a mole), so multiplying this number by the number of moles will give you the number of particles. In this case, multiplying by Avogadro's number will give you the number of dihydrogen gas molecules.