Question

5.0g of magnesium metal reacted with 1000cm3 of 4.5M of HCl solution to produce hydrogen gas. Calculate the amount of HCl left and the number of hydrogen molecules produced

Answer 1

This is a limiting reagent problem. You know that because amounts are given for both reactants. Here is a worked example of a limiting reagent problem. Just follow the steps.
http://www.jiskha.com/science/chemistry/limiting_reagent_problem.html