The amount of reactant in excess, or chemical left over after a completed reaction, is governed by the other reactant, which is completely used up and can react no more. Knowing the reactant in excess helps to ensure that you can successfully compute the final amounts of reactant and product, as well as ionic concentrations. In addition, computing the exact amounts of each chemical in advance of mixing them ensures that you achieve a complete reaction of all materials in the mix. If you know the percentage of excess for one chemical, you can easily use that information to add the correct amount of the other to complete the reaction.
Balance the Reaction
Balancing the chemical reaction provides the tools to assess exactly how much of each reactant is required to complete the desired reaction. For example, in the reaction Mg(OH)2 + HCl -->MgCl2 + H2O, the starting and finishing materials are out of balance. There is one magnesium atom on each side, but three hydrogen atoms on the left to two atoms on the right, one chlorine atom on the left to two atoms on the right, and two oxygen atoms on the left to one atom on the right. Since the magnesium is balanced, do not add a number to the left of the magnesium hydroxide. Adding a "2" in front of the hydrochloric acid and a "2" in front of the water should balance the equation. Now the equation has one magnesium atom, two chlorine atoms, two oxygen atoms and four hydrogen atoms on each side of the arrow, so the reaction equation is balanced: Mg(OH)2 + 2HCl --> MgCl2 + 2H2O
Calculate the Moles
To simplify the later math, reactant quantities are converted to moles. For example, to compute the excess reactant in a solution of 65.0 grams of magnesium hydroxide and 57.0 grams of hydrochloric acid, you would start by converting to moles. Use a periodic table to find the atomic mass units for each element. In this case, magnesium has 24.305 atomic mass units, oxygen has 16 and hydrogen has 1. You have one magnesium atom, two oxygen atoms and two hydrogen atoms, so 24.305 + (16 x 2) + (1 x 2) = 58.305 atomic mass units is the weight of a molecule of magnesium hydroxide. The molecular weight is then divided into the total grams -- 65 grams/58.301 atomic mass units -- to get 1.11 moles of magnesium hydroxide. The moles of hydrochloric acid are easier to compute. With only one atom each of hydrogen and chlorine, we get 1 + 35.45 = 36.45 atomic mass units; 57 grams/36.45 atomic mass units = 1.56 moles of hydrochloric acid.
Use Equation Ratios
Once the total moles are computed -- in this case, 1.56 moles of hydrochloric acid and 1.11 moles of magnesium hydroxide, the balanced equation Mg(OH)2 + 2HCl --> MgCl2 + 2H2O is used to determine how many moles of each reactant are required for the reaction to work. Two moles of hydrochloric acid are required to completely react with 1 mole of magnesium hydroxide, so divide the 1.56 moles of hydrochloric acid by 2. If the answer is greater than 1.11, the number of moles of magnesium hydroxide, the hydrochloric acid is in excess. If the answer is less than 1.11, the magnesium hydroxide is in excess. The example yields 1.56 moles/2, or 0.78 moles of hydrochloric acid, so the magnesium hydroxide is in excess and the hydrochloric acid is the limiting reactant.
Take a ratio of the moles of hydrochloric acid to moles of magnesium hydroxide to determine the percent of magnesium hydroxide that reacted: 0.78 moles/1.11 moles = 0.704, so 70.4 percent of the magnesium hydroxide was consumed. Then multiply the original amount by 70.4 percent to find the grams of magnesium hydroxide consumed. We get 65.0 * 0.704 = 45.78 grams consumed. Finally, subtract the amount consumed from the original amount: 65.0 grams - 45.78 grams = 19.21 grams. This is the amount of magnesium hydroxide in excess of the amount needed to completely react with the hydrochloric acid.