How many liters of hydrogen gas at stp are produced from the reaction of 50.0g of magnesium and 75g of hydrochloric acid

Here's how to do it: 

Balanced equation first: 
Mg + HCl = H2 + MgCl2 unbalanced 
Mg + 2 HCl = H2 = MgCl balanced 
Therefore 1 mole Mg reacts with 2 moles Hcl. 
50g Mg = ? moles (a bit over 2; you work it out) 
75 g HCl = ? moles (also a bit over 2; you work it out) 
BUT, you need twice the moles HCl; therefore it is the Mg that is in excess. (you can now work out how many moles are in excess, and therefore how much mg is left over). 

So, 2 moles HCl produce 1 mole H2(g) 
therefore, the amount of H2 produced is half the number of moles of HCl 
At STP, there are X litres per mole of gas (look it up - about 22 from memory) 
Therefore, knowing the moles of H2, you can calculate the volume