When a reaction is carried out in a laboratory, the reactants may not always be present in the perfect stoichiometric ratio, as shown by the balanced equation. A system may have an excess of one reactant or the other. The reactant that is completely used up in a reaction, which determines the amount of product that can form, is the limiting reactant (or limiting reagent). The reactant that is not completely used up in a reaction is the excess reactant (or excess reagent).
The steps to identify the limiting reactant are:
1. Determine the number of moles of the first reactant, and calculate the moles of the product that could form from it.
2. Determine the number of moles of the second reactant, and calculate the number of moles of the product that could form from it.
3. Compare the number of moles of the product that each reactant can form. The reactant that produces fewer moles of product is the limiting reactant.
4. Calculate the theoretical yield from the number of moles of product the limiting reactant can produce.
5. To determine the amount of excess reactant that will remain after the reaction is complete, compare the ratios of the limiting and excess reactants, calculate the moles of excess reactant used to convert to mass. Subtract the value from the initial mass of the excess reactant.
To calculate the number of moles of excess reactant after the reaction is complete, notice that the balanced equation shows that the mole ratio of CuO to H2SO4 is 1:1. Therefore, 0.07239 mol CuO is used in the reaction.Subtract this amount from the initial amount of CuO (0.1066 mol) to determine the number of moles of excess CuO.