How much can I make?
Maximizing Chemical Quantities
Limiting and Excess Reactants
Percent Yield
What is a limiting reactant?
• Limiting Reactant - reactant in a chemical reaction
that limits the amount of product that can be
formed
• The reaction will stop once all of the limiting
reactant is consumed.
What is an excess reactant?
• Excess Reactant - reactant in a chemical reaction
that remains when a reaction stops
• The excess reactant remains because there is
nothing to react with it!
How many cars can be made?
No matter how many tires there are, if there are only 8 car bodies, then only 8 cars can be
made.
Example #1:
Copper metal reacts with sulfur to form copper (I)
sulfide according to the balanced equation.
2
1
1
_____
Cu + ____
S  _____
Cu2S
What is the limiting reactant when 80 g Cu reacts with 25 g S?
1 mol Cu
80 g Cu x _________
= 1.26 mol of Cu
63.5 g Cu
1 mol S =
25 g of S x _______
32.1 g S
1.09 mol of S
Example #1:
2 Cu + 1 S  1 Cu2S
• What is the mole ratio of Cu to ? ____
2 Cu : ____
1 S
1 mol S = 0.63 mol of S
• 1.26 mol of Cu x ___________
2 mol Cu
Translation:
• I only need 0.63 mol of S to fully react with copper.
1.09 mol of S.
• But I was given ________
• Excess = Sulfur
Limiting = Copper
What’s the max amount of product that I
can yield?
• Use Stoichiometry to determine the max amount
of product that can be formed.
1 mol Cu2S x ________
159.1 g Cu2S =
1.26 mol Cu x _______
2 mol Cu
1 mol Cu2S
100 g Cu2S
Theoretically vs. Actually
Percent Yield
• Theoretical Yield = maximum amount of product
that could be formed from a given
amount of reactants
• Actual Yield = amount actually formed when a
reaction is carried out
Why don’t reactions always go to
completion?
• purity of reactants
• loss of product formed
during filtration
• competing reactions
• measurement error
Theoretical Yield Example
• What is the theoretical yield of CaO if 24.8 g of
CaCO3 is heated?
_____ CaCO3  _____ CaO + ______ CO2
1 mol CaCO3 x _________
1 mol CaO x ________
56 g CaO = 13.9 g CaO
24. 8 g CaCO3 x ________
100.1 g CaCO3
1 mol CaCO3
• Theoretical Yield = 13.9 g CaO
1 mol CaO
Percent Yield Example
• What is the percent yield if 13.1 g of CaO was
actually produced when 24.8 g of CaCO3 was
heated?
• % Yield= 13.1 g x 100 = 94.2 %
13.9 g