After writing a balanced equation for the reaction between magnesium and hydrochloric acid, calculate the mass of magnesium needed to produce 90ml of hydrogen gas @ STP conditions.

Answer 1

Here is a worked example of a stoichiometry problem. Just follow the steps. Remember, L/22.4 will convert volume of H2 to moles H2.
http://www.jiskha.com/science/chemistry/stoichiometry.html