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The limiting reactant is the reactant that will run out first. This means there will be excess of the other reactant, making it the non limiting reactant. Most calculations will be based off of the limiting reactant because the excess of the non limiting reactant will not be used.
When finding the excess, you must find which reactant is limiting first. Once you've done this, you can then determine which reactants are in excess; i.e. there is some left over that isn't used in the reaction. An easy way to find how much excess is left over is to convert all of your reactants into moles of one reactant, for instance, if you had the reaction 2H2 + O2 -> 2H20, then convert your reactants into moles of H2 or O2 first. Whichever has more moles has the limiting reactant's # of moles subtracted from it, then you'll convert back and find how much you have excess.
The reactant that is in excess will not be consumed entirely in the reaction, whereas in theory the limiting reactant will be consumed entirely and there will be nothing left over. The amount of product formed is based on how many moles you have of the limiting reactant-- however many moles you have of the limiting reactant determines how many moles of product can be formed (along with the molar ratios between reactant and product). Hope this helps! :)
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