hydrogen gas can be produced in the laboratory through the reaction of magnesium metal with hydrochloric acid: mg(s)+2hcl(aq)→mgcl2(aq)+h2(g) when 21.0g of mg reacts, what volume, in liters, of h2 gas is produced at 20∘c and 760 mmhg ?

Answer 1

Look at the balanced equation. for each mole of Mg consumed, two moles of H2 gas are produced. So the question is, how many moles of Mg was used: 21g/atomicmassMg

Now, twice that is the moles of H2 gas.
Volume?

V=nRT/P use the correct R for your units.