First you identify the limiting reactant. Then you calculate the theoretical yield of product from the amount of the limiting reactant.
Aspirin is prepared by the reaction between ethanoic anhydride and salicylic acid.
ethanoic anhydride + salicylic acid → aspirin + ethanoic acid
C₄H₆O₃ + C₇H₆O₃ → C₉H₈O₄ + C₂H₄O₂
A + B → C + D
What is the theoretical yield of aspirin (C) if you reacted 4.32 g of
ethanoic anhydride (A) with 2.00 g of salicylic acid (B)?
Molar masses are
Ethanoic anhydride = A = C₄H₆O₃ = 102.1 g/mol
Salicylic acid = B = C₇H₆O₃ = 138.1 g/mol
Aspirin = C = C₉H₈O₄ = 180.2 g/mol
Mass of aspirin from A:
4.32 g A ×
Mass of aspirin from B:
2.00 g B ×
B gives the smaller amount of aspirin, so the B is the limiting reactant. Since B is the limiting reactant, the theoretical yield is
2.61 g of aspirin.