If 26.5 mL of 0.222 M HCl reacts with excess Mg, how many mL of hydrogen gas are produced at STP?

Answer 1

Mg + 2HCl ==> MgCl2 + H2
mols HCl = M x L = ?
Convert mols HCl to mols H2. That will be mols H2 = 1/2 mols HCl.
Then convert mols H2 to L at STP knowing that 1 mol H2 will occupy 22.4 L at STP, then convert that to mL.