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SCH 3U
Stoichiometry
What is Stoichiometry?
Stoichiometry is the study of the quantitative relationships among the amounts of reactants used and the
amounts of products formed in a chemical reaction. The basic tool of stiochiometry is a balanced chemical
equation. A balanced chemical equation is essential for making calculations and predictions related to
quantities in a chemical reaction.
For example: in cooking –a recipe is the balanced chemical reaction and the outcome of the recipe depends
on the quantities of reactants or starting ingredients. i.e. suppose you are making a turkey sandwich.
Mole ratios in balanced chemical equations
A balanced chemical equation tells us the ratio of the number of moles of reactants to products taking part in
a chemical reaction.
Therefore they tell us two important pieces of info:
1. type and number of atoms and molecules that interact and how they arrange.
2. the relative number of moles of atoms and molecules that interact and form.
Example
N2(g) + 3H2(g)  2NH3(g) …1 mole of N2 reacts with 3 moles of H2 to give 2 moles of ammonia (NH3)
The ratio is 1:3:2 – and this ratio is always maintained no matter what different amounts are actually used...
3 mol N2 : 9 mol H2 : 6 mol NH3
or
10 mol N2 : 30 mol H2 : 20 mol NH3
The ratio still remains 1:3:2, and this ratio is called the mole ratio.
Practice
Step 1. Balance the following equations.
Step 2. In the respective tables, conserve the balanced ratio based on the information provided.
1.
___ NaNO3
+
___ PbO

___ Pb(NO3)2 +
___ Na2O
3
5
2. ___ AgI
+
___ Fe2(CO3)3 
___ FeI3
+
___ Ag2CO3
3
7
3. ___ C2H4O2
+
___ O2

___ CO2
+
___ H2O
+
___ Li2SO4
10
3
0.25
4. ___ ZnSO4
+
___ Li2CO3
5
15

___ ZnCO3
5. ___ Mn(NO2)2 +
___ BeCl2

___ Be(NO2)2 +
___ MnCl2
3.5
6.85
6. ___ AgBr
+
___ GaPO4
___ Ag3PO4

+
___ GaBr3
12
13
7. ___ H2SO4
+
___ B(OH)3

___ B2(SO4)3
+
___ H2O
5
12
8. ___ Fe
+
___ AgNO3

___ Fe(NO3)2 +
___ Ag
2.25
0.25
Mole to Mole Stoichiometry
The following technique can be used to predict the # of moles that will react or form in an equation.
1.
2.
3.
4.
Identify the given number of moles.
Identify the balance of the Given.
Identify the balance of the Desired (substance you wish to find).
Multiply.
Moles of given(n) X
Balance of Desired
Balance of Given
 Moles of Desired (n)
Using the balanced equation below predict, the number of moles of propane that react to produce 8.4
moles of carbon dioxide gas.
___ C3H8 (g)
+
___ O2 (g) 
___ CO2 (g) +
___ H2O
More practice!
For each of the following questions:
i.
- write out the following reactions,
ii.
- balance them and then answer the respective questions.
iii.
- be sure to maintain the coefficient ratio in the balanced equation.
1. Sodium sulfate reacts with carbon to form sodium sulfide and carbon dioxide.
How many moles of carbon are needed to completely react with 5.15 moles of sodium sulfate?
2. Nitrogen dioxide reacts with water to form nitric acid and nitrogen monoxide. How many moles of nitric
acid are produced when 0.75 moles of nitrogen dioxide are completely reacted?
3. Balance the following reactions and then answer the respective questions. Be sure to maintain the
coefficient ratio in the balanced equation.
a. ___ Cl2 (aq) + ___ H2O (l) → ___ HCl (aq) + ___ HOCl (aq)
How many moles of water are needed to produce 2.5 moles of hydrochloric acid?
b. ___ PCl3 (aq) + ___ H2O (l) → ___ H3PO3 (aq) + ___ HCl (aq)
How many moles of hydrochloric acid are produced from 2.17 moles of phosphorus trichloride?
c. ___P2H4 (l) → ___ PH3 (g) + ___ P4 (g)
How many moles of diphosphorus tetrahydride are needed to create 3.15 moles of elemental phosphorus?
d. ___C3H8 (g) + ___ O2 (g) → ___ CO2 (g) + ___ H2O (l)
How many moles of oxygen are required to react with one mole of propane?
Stoichiometry Problems
The mole ratio of any two substances in any balanced chemical equation consists of simplest whole numbers.
However, the mass ratio of any two substances in an actual chemical reaction rarely consists of whole
numbers. If you know the amount in moles, number of particles, or mass in grams of any substance in a
chemical reaction, you can calculate the amount, number of particles, or mass in grams for any other
substance in the chemical reaction.
Mole ratio  mass
Using the mole ratio we can get mass relations (the mole ratio cannot be directly applied to masses, i.e. there
is technically no mass ratio!)
Example 1
What mass of hydrogen would react with 50.0 g of nitrogen to produce ammonia?
Step 1: Determine the equation for the reaction and balance the equation
N2(g) + 3H2(g)  2NH3(g)
Step 2: find the n (# of moles) of nitrogen (convert mass  moles)
Step 3: use the mole ratio (from the balanced chemical equation) to find n (# of moles) for hydrogen
Step 4: convert n of hydrogen to mass of hydrogen (convert moles  mass)
Example 2
If 50.0 g of aluminum is added to excess silver nitrate, what mass of Ag would result?
read p.223 - 226
Q# p. 227 1 - 6
(stoichiometry – answers provided)
pg. 228 # 8 pg. 229 #4
Mass to Mass Stoichiometry
We can use a simple three step method to solve stoichiometric questions with balanced equations.
1. Identify the given and convert it to moles.
2. Identify the desired, and multiply the given number of moles by the mole ratio or equation
factor, to produce moles of desired substance.
3. Convert moles of desired substance to the units asked for in the question.
Example: What mass of methane gas (grams) must burn to produce 365 grams of water?
____CH4 (g) + ____ O2 (g)  ____H2O(g) + ____ CO2(g)
More Practice
1.
Several brands of antacid tablets use aluminum hydroxide to neutralize excess acid.
Al(OH)3(s) + 3 HCl(aq)  AlCl3(aq) + 3 H2O(l)
[Molar masses:
78.01
36.46
133.4
18.02]
(a) What quantity of HCl, in grams, is required to react with a tablet containing 0.750 g of Al(OH) 3?
(b) What quantity of water, in grams, is produced?
2.
If 10.0 g of carbon is combined with an exact, stoichiometric amount of oxygen to produce carbon dioxide, what
mass, in grams, of CO2 can be obtained? That is, what is the theoretical yield of CO 2?
[Molar masses: C:
12.011 O2: 32.00 CO2: 44.01]
3.
The equation for one of the reactions in the process of reducing iron ore to the metal is
Fe2O3(s) + 3 CO(g)  2 Fe(s) + 3 CO2(g)
[Molar masses:
159.7
28.01
55.85
44.01]
(a) What is the maximum mass of iron, in grams, that can be obtained from 454.0 g of iron(III) oxide?
(b) What mass of CO is required to reduce the iron(III) oxide to iron metal?
4.
Burning coal and oil in a power plant produces pollutants such as sulfur dioxide, SO2. The sulfur-containing
compound can be removed from other waste gases, however, by the following reaction:
2 SO2(g) + 2 CaCO3(s) + O2(g)  2 CaSO4(s) + 2 CO2(g)
[Molar masses: 64.07
100.1
32.00
136.2
44.01]
(a)
(b)
(c)
5.
Name the compounds involved in the reaction.
What mass of CaCO3 is required to remove 155.0 g of SO2?
What mass of CaSO4 is formed when 155.0 g SO2 is consumed completely?
Your body deals with excess nitrogen by excreting it in the form of urea, NH2CONH2. The reaction producing it is
the combination of arginine (C6H14N4O2) with water to give urea and ornithine (C5H12N2O2).
C6H14N4O2 + H2O  NH2CONH2 + C5H12N2O2
[Molar masses:
174.2
18.02
60.06
132.2]
(a) If you excrete 95.0 mg of urea, what mass of arginine must have been used?
(b) What quantity, in grams, of ornithine must have been produced?
6.
Gold will dissolve in the acid mixture known as aqua regia according to the following reaction;
____ Au + ____HNO3 + ____ HCl  ____ AuCl3 + ____ NO + ____ H2O
How much gold (III) chloride would be produced if 250 grams of HCl is consumed in the reaction?
Answers: 1. a) (1.05 g) b) (0.519 g)
2. (36.6 g)
3. a) (317.6 g) b) (238.9 g)
4. a) sulfur dioxide and calcium carbonate and oxygen react together to produce calcium sulfate and carbon dioxide
b) 242.2 g c) 329.6 g
5. a) (0.276 g)
b) (0.209 g)
c) 6. 693 g