What equation do I use to do the following problem? (solutions & molarity)?

Question

A 225 mL sample of HCl solution reacts with excess Mg to produce 14.0 L H2 gas at STP. What is the molarity of the HCl solution?

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Please explain which equation to use/how to work the problem. If you feel like it, it would be helpful to also provide the answer, so I can double check things.

Answer 1

14/22.4 x 2 x 1000/225