Always start with a balanced equation. Use stoichiometry to calculate how much product is produced by each reactant. (It doesn’t matter which product is chosen but the SAME product MUST be used for BOTH reactants so that the amounts can be compared). The reactant that produces the lesser amount of product is the limiting reagent. To find the excess reactant, calculate how much of the excess reagent actually did react with the limiting reagent and subtract from the amount in the original sample.
Example)If 4.95g of ethylene(C2H4) are combusted with 3.25g of oxygen, what is the limiting reagent? How many grams of each product are formed? What mass of the excess reactant remains once the reaction is complete?
C2H4 + 3O2 = 2CO2 + 2H2O – 4.95gC2H4 x (1molC2H4/28gC2H4) x (2molCO2/1moleC2H4)x (44gCO2/1molCO2) = 15.6gCO2…C2H4 is the excess reagent
– 3.25gO2 x (1molO2/32gO2) x (2molCO2/3molO2) x (44gCO2/1molCO2) = 2.98gCO2…O2 is the limiting reagent
– 3.25gO2 x (1molO2/32gO2) x (2molH2O/3molO2) x (18gH2O/1molH2O) = 1.22gH2O is produced
– 3.25gO2 x (1molO2/32gO2) x (1molC2H4/3molO2) x (28gC2H4/1molC2H4) = .948gC2H4…4.95
– .948 = 4.00gC2H4 of excess reagent is left over after the reaction is complete