Chemistry HW Help Please (Stoichiometry)

Ok well my chemistry teacher sucks at explaining things throughly, none of my peers understand it either (if they did, I'd get help from them before coming here). I understand how to balance the chemical equation but the rest is just confusing.

2. Mg(s) + 2HCl(aq) ---> MgCl2(aq) + H2(g)

Determine the mass of H2(g) that can be produced from the following.

a. 10.0g Mg(s) reacts completely with HCl(aq)
b. 20.0g HCl(aq) reacts completely with Mg(s)

a. 0.823 g H2
b. 0.548 g H2

Mg - 24.3 g/mol
HCl - 36.5 g/mol

To determine the answer for a, you will have to divide the given mass by the compound/element's molar mass, which is listed above. Then you will have the amount of moles which you will need to multiply by the mole ratio. The mole ratio for Mg:H2 is 1:1, so the number of moles doesn't change. You then multiply that number by the molar mass of H2, which is 2.0, giving you 0.823 g H2.

To determine the answer for b just follow the same steps but this time the mole ratio for HCl:H2 is 2:1, so divide the number of moles by 2 then multiply that number by the molar mass of H2, giving you 0.548 g H2.

It's been a while since I've been in a chemistry class but I do remember being taught an easy way to remember how to do this. Think of it like a mole hill. You have to go over it to solve the problem. At the bottom of one side is the mass, which is in grams. Then to get to the top you have to be in moles. To do that you convert grams to moles. The atomic mass can be found on the periodic table. That gives you grams per mole, so therefore you would divide the mass by this number. Then you would use the coefficients that you used to balance the equation for the molar ratios. So for part a it means that for every mole of Mg that you have you get 1 mole of H2 and for part b it means for every 2 moles of HCL you have you get 1 mole of H2. Now that you're done with the top of the mole hill you must come back down to the bottom, so to do that you have to convert from units of moles to units of mass, so again, you would use the atomic mass from the periodic table to convert the number of moles H2 to grams of H2.