Step 2 Now that we have a balanced equation, we can figure out what the weight ratios are. We could figure it out for 1 or 2 moles of reactants or products, but lets not do that. Instead, lets use the weights yo are given and convert them to numbers of atoms
72g of HCl is how many moles? (We use the number "moles" because if we asked how many atoms are in HCl the number would be so big that we'd spend too much time just writing it down and we'd be very likely to skip a zero or make some other kind of mistake. So, chemists use the number "mole" just like bakers use the number "dozen". One mole of HCl is made up of one mole of H and one mole of Cl atomic weights are 1.01 and 35.45, respectively. SO the weight (mass actually) of a mole of HCl is 1.01+35.45 = 36.46g But lets only use two significant figures and say 1+35=36. So 72 grams is 2 moles of HCl. Looking at the equation we see two moles of HCl will react with one mole of Mg. What is the mass of a mole of Mg? It is 24grams. Wait a minute! The reaction only involves 3.5 grams of Mg. That means that the 72 g or HCl is WAY too much. So we have figured out that after the reaction is over there will be a LOT of HCl unreacted. So lets figure out how many moles of Mg there are: 3.5/24 = 0.15 moles of Mg
OK so we now know what the limiting reactant is. Given 0.15 moles of Mg how much H2 will be produced? Well, the number ratio is in the balanced equation: 2:1→1:1
0.15 moles of Mg will produce 0.15 moles of H2 ← note this is H2 not H !!!!
So, we now know how many moles of H2 (gas) we will get.
Using the ideal gas law PV=nRT we now know n. ANd of course P and T are defined by STP, R is a constant so all you have to do is solve for V. Plug and chug.