Assuming you react 221g of Zn metal with 2 Equivalents of HCl, you will get 6.81g of H2(g).
Zn(s) + 2 HCl(aq) --> ZnCl2(s) + H2(g)
First, calculate moles of Zinc. Moles = mass/molar mass; moles = 221g /65.409 g per mole; moles = 3.3787
Second, look at the stoichiometric ratio between Zinc and Hydrogen gas. The ratio is 1:1 so there are 3.3787 moles of hydrogen gas.
Third, calculate mass of hydrogen gas. Mass = moles x molar mass; mass = 3.3787 moles x 2.0158 g per mol. Mass hydrogen gas = 6.81g
