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