How do you calculate the number of mol corresponding to 6.2 g #H_2#?
Divide the given mass (6.2 g) by the molar mass of hydrogen gas, which is 2.0 g, and you get 3.1 moles.
You must note that hydrogen gas (like oxygen, nitrogen, chlorine and others) exists in the gas phase in the form of diatomic (two-atom) molecules.
So, if you look up the molar mass of hydrogen on a periodic table, you must multiply by two (the number of hydrogen atoms in each molecule) to obtain the mass of one mole of hydrogen gas.
Then as you would for any molecule, divide the given mass by the molar mass to get the number of moles of that substance.