Question

What is the maximum amount of magnesium chloride you can produce when 15 g of magnesium (Mg) reacts with 20 g of hydrochloric acid (HCl)?

Mg + HCl → MgCl2 + H2

Answer 1

This is a limiting reagent (LR) problem.
Write and balance the equation. You have that but it isn't balanced.
Mg + 2HCl ==> MgCl2 + H2

Convert what you have top mols
mols Mg = 15/24.3 = approx 0.6 but you will need to ALL of these calculations over since I've estimated all of them.
mols HCl = 20/36.5 = about 0.5

Convert mols of each reactant to mols of product using the coefficients in the balanced equation.
0.6 mols Mg x (1 mol MgCl2/1 mol Mg) = about 0.6
0.5 mols HCl x (1 mol MgCl2/2 mol HCl) = about 0.25.
In LR problems the SMALLER number always wins so HCl is the LR.

Using the LR, calculate grams product formed.
about 0.25 mols x molar mass MgCl2 = grams MgCl2 produced.

Post your work if you get stuck.