what volume of hydrogen gas, measured at STP, will be produced when 16.0g of magnesium reacts completely with an excess of hydrochloric acid

Answer 1

Mg + 2HCl = MgCl2 + H2

So, each mole of Mg produces 1 mole of H2

So, how many moles of Mg is 16g?

1 mole of H2 occupies 22.4L at STP.