Let's create an imaginary problem, shall we?
100grams Propane (C3H8) combines with excess oxygen to yeild 238.168grams CO2 and a helluvalotta water.
What is the theoretical yeild of CO2? What is the percent yeild?
1. Determine what element, ion, or component can be used to compare two substances with known quantity. This rules oxygen out, as we have no friggin' idea how much we used, and this rules hydrogen out, because "helluvalottawatta" is not a quantitative measurement. Therefore, we will use carbon as our reference, and compare the 100 grams Propane to the 238.168 grams CO2.
2. Here comes the dimensional analysis, the chain of conversions that resembles that little unthreatening can of worms (except watch out when opening it, that lid's sharp) but soon all them worms pile on top of each other to create ONE BIG WORM... and... and KILL YOU with it's WORMLIKE VERACITY!
So anyway, place your 100 grams of propane at left, convert to moles of propane(Molar weight: 44.09652gC3H8/mol), convert to moles of carbon(3mol carbon in 1mol propane), find how many moles of carbon dioxide this can THEORETICALLY create(each mol of carbon can pile two moles of oxygen on it and make one mol of CO2, so it's a one-to-one conversion) and finally, find how much all this CO2 will weigh(molar weight of CO2 is 44. 0098, almost the same as propane, which is just beautiful)
3. VOILA! you now have the weight, in grams of CO2, of your theoretical yeild.
4. Simple. For the percent yeild, divide the yeild provided by the theoretical one you determined. It better be less than 100%. Keep in mind that this percent yeild applies to the entire equation, not just the product you used to calculate it. This percent determines how much of the reaction actually took place, and wasn't lost due a a host of factors such as by sticking to the little spatula thingy.
Okay, so as long as you don;t get sidetracked and start thinking about cans of worms and maybe what would happen if you were in hand-to-hand combat with a coyote, badger, and/or huge-ass pelican, you can use those steps to do any test question on single-reaction yeilds.
Note: a lot of people don;t need to think of individual steps like "well now i'm converting to moles..." etc. whilst doing dimensional analysis. If you are one of these lucky people, ignore the stuff i wrote in the latter half of step two that wasn;t talking about worms. However, if you are like me, make sure you know what your are trying to do and accomplish in your analysis, don;t just start with reactant and end with product. Even though it works, it just... it will fail you inevitably.
If you wanna see how your skillz serve you, try some prolems like pg. 130 #107, or pg. 129 #90.
For all that wondered why i didn;t exlain this BEFORE the test, no one really told me to. Just post to this community with a question like "d00d, how in the bloody hell do i know what the limiting reactant is?" or "in a battle between a beaver and an albatross, which do you think would win?" and i'll "hit dat shit!" so to speak... if by "hit dat shit!" you mean offer assistance in understanding the concepts involved.