5.3g of magnesium reacts with excess HCI. How many liters of hydrogen gas will be produced?

This four step process will do volumetric or mass stoichiometry problems all day long.

1. Write and balance the equation.
Mg + 2HCl ==> MgCl2 + H2

2. Convert what you have (in this case grams Mg) to mols. mols = grams/molar mass. That's approx 0.2 but you should do it more accurately.

3. Using the coefficients in the balanced equation convert mols of what you have (Mg) to mols of what you want (in this case H2). That's
0.2 mol Mg x (1 mol H2/1 mol Mg) = 0.2 x 1/1 = 0.2 mol H2 produced.

4a. If you want volume, then mols H2 x 22.4 L/mol (at STP) = ? L H2.
4b. If you want other conditions than STP, use PV = nRT.
4c. If you want grams, then g = mols x molar mass.

Isn't chemistry fun?