Chemistry 11
The Limiting Reactant
Mrs. Chantelle MacCuspic
A balanced chemical equation shows the mole ratios of the reactants and products. To emphasize this,
the coefficients of equations are sometimes called stoichiometric coefficients. Reactants are said to
be present in stoichiometric amounts when they are present in a mole ratio that corresponds exactly to
the mole ratio predicted by the balanced chemical equation. This means that when a reaction is
complete, there are NO reactants left.
However, during most reactions, it is virtually impossible to obtain 100% yield in a chemical reaction by
combining the reactants in exact proportions. In order to increase the odds that at least one reactant
will react completely, we often add more than is needed of another reactant. This reactant is said to be
in excess. The reactant that is completely used up in the reaction is called the limiting reactant or
reagent because it limits the amount of product formed. In other words, the limiting reactant
determines the amount of product formed in the reaction. Therefore, when given the mass of two
reactants it is necessary to determine the limiting reactant BEFORE you can calculate how much product
will be formed.
For example, in the first step in extracting zinc from zinc oxide: ZnO(s) + C(s)  Zn(s) + CO(g) ,
in an industrial setting (where being exact isn’t always necessary), engineers add more carbon, in the
form of charcoal, than is necessary for the reaction. This means that ALL the zinc oxide reacts, but
there is carbon left over.
Limiting Reactant Exercises
1. Ammonia (NH3) can be formed from the elements N2 and H2, as shown below.
N2 + 3H2  2 NH3
a. How many moles of ammonia can be made from one mole of N2 and 3 moles of H2?
b. Suppose we had 12 moles each of the N2 and H2 available to react. How many moles of ammonia could
we make? Which of the reactants would be limiting? Would any of the reactants be left over? How
many moles?
2. a) What mass of magnesium chloride can be produced when 6.02 x 1023 molecules of hydrochloric acid
are added to 3.00 g of magnesium?
b) What is the volume of hydrogen at STP? (Use the limiting reactant)
3. The fizz produced when an Alka-Seltzer tablet is dissolved in water is due to the reaction between
sodium bicarbonate and citric acid.
3NaHCO3(aq) + H3C6H5O7(aq)  3CO2(g) + 3H2O(i) + Na3C6H5O7(aq)
In a certain experiment 1.00 g of sodium bicarbonate, NaHCO3, and 1.00 g of citric acid, H3C6H5O7,
are allowed to react.
a) How many molecules of carbon dioxide is formed at STP?
b) Which reactant is the limiting reactant?
c) How much of the excess reactant remains after the limiting reactant is completely consumed?
LIMITING REACTANT QUESTION
Consider the double replacement reaction between sodium iodide and lead (II) nitrate. One of the
products, lead (II) iodide is a bright yellow insoluble substance. Prior to the 1960’s it was used as a dye
in yellow paint. Lead poisoning could result if you eat the paint so is different dye is used in today’s
paint.
A) Write a balanced chemical reaction.
B) If 10.0 grams of sodium iodide react with 50.0 mL of a 2.2 mol/L solution of lead (II) nitrate, how
many grams of lead (II) iodide are produced?