Question

Magnesium reacts with HCl to produce magnesium chloride and hydrogen gas.
Mg(s)+2HCl(aq)->MgCl2(aq)+H2(g).
What is the molarity of the HCl solution if 330 ml of the HCl solution reacts with magnesium to produce 7.40 L of H2 gas measured at STP?

Answer 1

7.40 L H2 gas @ STP = 7.40/22.4 = moles H2 @ STP.

moles HCl = moles H2 x (2 moles HCl/1 mole H2) = ??moles H2 x 2 = xx moles HCl

Then M HCl = mols HCl/L HCl
You have moles and L, solve for M.