How to do a stoichiometry/limiting reagent problem:
1. Write a balanced chemical equation.
2. Convert a known amount to moles.
3. Using the relationships in the balanced equation, convert the moles of the known substance to moles of the substance you're looking for.
4. Convert the moles of the substance you're looking for into whatever quantity you'd like to know.
Those are the same 4 steps you'll use for every stoichiometry problem you will ever do. The details will change, but the basic steps are the same.
12.5grams of nitrogen gas reacts with 11.6g of hydrogen gas to form ammonia gas. What is the theoretical yield of ammonia gas in grams?
Step 1: N2(g) + 3 H2(g) --> 2 NH3(g)
If nitrogen is the limiting reagent...
Step 2: (12.5g N2) / (28.014g/mol) = 0.4462mols N2(g)
Step 3: (0.4462mols N2(g)) (2 mols NH3(g) / 1 mol N2(g)) = 0.8924mols NH3(g)
Step 4: (0.8924mols NH3(g)) (17.031g/mol) = 15.2g NH3(g)
But we don't really know if nitrogen is the limiting reagent, so...
If hydrogen is the limiting reagent...
Step 2: (11.6g H2) / (2.0158g/mol) = 5.7545mols H2(g)
Step 3: (5.7545mols H2(g)) (2 mols NH3(g) / 3 mol H2(g)) = 3.8364mols NH3(g)
Step 4: (3.8364mols NH3(g)) (17.031g/mol) = 65.3g NH3(g)
Since the nitrogen gas is all used up by the time we've made 15.2g of ammonia, there's no way we could make 65.3g of ammonia, so nitrogen must be the limiting reagent and the theoretical yield is 15.2g of ammonia gas.
What can you do besides practice, practice, practice stoichiometry problems to get better at them? Always, always, always write out your units and make sure they cancel out correctly. If your units to not cancel out correctly, it is exceedingly unlikely that you are setting up the problem correctly. Units are a wonderful little gift that lets you check your own work, I use them for every problem I set up.
Good luck and practice, practice, practice, practice, practice, practice, practice, practice, practice.
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