If you're given information about the reactants in a chemical reaction, and need to find out something about the products, you'll likely have to figure out which reactant will run out first. Chemists call the reactant that runs out first the limiting reagent and you need to be able to find it. It's easier than you might think ... just follow these steps!
Q: How much CO2 and H2O is formed (in g) when 3 g of CH4 react with 5 g of O2?
Step 1 - Create a balanced chemical equation
Step 2 - Calculate the number of moles of EACH reactant
Step 3 - Figure out which reactant is limiting (will run out FIRST)
TRICK: Divide the number of moles of each by the coefficient in the balanced chemical formula. The substance that gives the smallest number this way is the limiting reagent. This works because, in the example we're working on, O2 is consumed TWICE as fast as CH4 ... so we need double as much O2 as CH4 !
So, O2 is the limiting reagent.
Step 4 - Figure out how much of each product forms (which are you asked about?)
O2 is the limiting reagent, so we consume all 0.15625 mol of it. This is our benchmark. Multiply this number by the coefficient of the product you're solving for, and divide by the coefficient of the reactant that is limiting. Alternatively, look at the last two calculations here...
Step 5 - Convert to whatever units you need
Often, this means converting them to "mass":