Stoichiometry
Chemical Equations
Reactants are written on the left side of the arrow and products are written on the right side of the
arrow.
The Law of Conservation of Mass tells us that the number of atoms of a given element on the
reactant side of the arrow must equal the number of atoms of that same element on the product side
of the arrow.
The coefficients of a balanced chemical equation tell us how many of each species are involved in
the reaction.
When balancing a chemical equation, you must not change the identities (the subscripts) of any of
the species; you may only change how many (the coefficients) of the species participate.
Ex. 1
Balance the following chemical equations:
____ NaBr + ____ H3PO4
____ C3H5N3O9
____ Ca(OH)2
+


____ HBr
____CO2
____ SO2
+
+
+
____ N2
____ O2
1

____ Na3PO4
+
____ H2O
+
____ O2
____ H2O + ____ CaSO4
2
Tips for Balancing Chemical Equations:
We have been learning how to balance chemical equations. The steps for balancing equations are
outlined in your textbook. There are, however, a few tricks which you can apply to balancing
chemical equations.
The first involves a quick inspection to determine if you have an element that appears in only one
compound on both sides of the equation and has subscripts like 2 and 3, 4 and 5, etc. If so, then try
making the subscripts the coefficients, but switch sides of the equation. For example:
____ Fe + ____ Cl2 
____ FeCl3
In this reaction, Cl appears only once on the left side of the reaction and has a subscript of 2. It
appears only once on the right side of the reaction and has a subscript of 3. A good first step for
balancing an equation like this would be to make 3 (the subscript for Cl on the right side of the
equation) the coefficient in front of Cl2 on the left side of the equation, and make 2 the coefficient
in front of FeCl3 on the right side of the equation. You then have:
____ Fe + 3 Cl2 
2 FeCl3
All that’s left is to balance the Fe:
2 Fe + 3 Cl2 
2 FeCl3
Another trick that will help you in counting is to count polyatomic ions as units if they appear on
both sides of the equation:
____ KCl + ____ HNO3 + ____ O2  ____ KNO3 + ____ Cl2 + ____ H2O
Here you can count the “NO3” as a unit instead of counting out the individual N’s and O’s. Set
HNO3 to have a coefficient of “1”:
____ KCl + 1 HNO3 + ____ O2  ____ KNO3 + ____ Cl2 + ____ H2O
Now, you have 1 NO3 group on the left side of the equation and 1 NO3 group on the right side of
the equation:
____ KCl + 1 HNO3 + ____ O2  1 KNO3 + ____ Cl2 + ____ H2O
Balance the rest as usual:
4 KCl + 4 HNO3 + O2  4 KNO3 + 2 Cl2 + 2 H2O
3
For more information on balancing chemical equations see
http://www.chem.uic.edu/chem112/FALL02/bal_eqn.pdf
4
Patterns of Chemical Reactivity
Using Periodic Trends
Combustion in Air
Ex. 2
Write a balanced chemical reaction for the combustion of octane, C8H18(l), in air.
Combination and Decomposition
5
Ex. 3
Write the balanced chemical equation for the combination of the metallic element
calcium with the nonmetallic element oxygen, O2.
Atomic and Molecular Weights
Atomic Mass
Molecular Weight
Ex. 4
Calculate the molecular weight of
(a)
N2O5
(b)
Ca(C2H3O2)2
Percentage Composition
6
7
Ex. 5
Calculate the percentage by mass of oxygen in 4a and 4b above.
The Mole
Ex. 6
Calculate the number of O atoms in 0.470 mol of C6H12O6.
Molar Mass
Ex. 7
What is the mass in grams of 1 mol of glucose, C6H12O6?
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How to interconvert between grams, moles and number of molecules
Ex. 8
How many moles of chloride ions are in 0.0750 g of magnesium chloride?
Ex. 9 What is the mass, in grams, of 1.75  1020 molecules of caffeine, C8H10N4O2?
Ex. 10 What is the molar mass of cholesterol if 0.00105 mol has a mass of 0.406 g?
Empirical Formulas from Experiment
9
Ex. 11 Determine the empirical formula of a compound containing 0.104 mol K, 0.052 mol C,
and 0.156 mol O.
10
Molecular Formula from Empirical Formula
Ex. 12 What is the molecular formula of a compound that has an empirical formula, CH2,
and
a molar mass of 84 g mol–1?
11
Combustion Analysis
Ex. 13 Combustion analysis of toluene, a common organic solvent, gives 5.86 mg of CO2 and
1.37 mg of H2O. If the compound contains only carbon and hydrogen, what is its
empirical formula?
Stoichiometry of Balanced Equations
12
grams
reactant
moles
reactant
moles
product
grams
product
Ex. 14 Propane, C3H8, is a common fuel used for cooking and home heating. What mass of
O2 is consumed in the combustion of 1.00 g of propane?
Limiting Reactants
The reactant that is completely consumed, or used up, in a chemical reaction is called the limiting
reactant.
Ex. 15 When hydrogen sulfide gas is bubbled into a solution of sodium hydroxide, the
reaction forms sodium sulfide and water. How many grams of sodium sulfide are
formed if 2.50 g of hydrogen sulfide is bubbled into a solution containing 1.85 g of
sodium hydroxide, assuming that the limiting reactant is completely consumed?
13
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Theoretical and Percentage Yield
Ex. 16 When ethane, C2H6, reacts with chlorine, Cl2, the main product is C2H5Cl, but other
products are also obtained in small quantities. The formation of these other
products
reaction of
reduces the yield of C2H5Cl. Calculate the percent yield of C2H5Cl if the
125 g of C2H6 with 255 g of Cl2 produced 206 g of C2H5Cl.
15