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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
3
7
+
___ Ag2CO3
3. ___ C2H4O2
+
___ O2

___ CO2
+
___ H2O
+
___ Li2SO4
10
3
0.25
4. ___ ZnSO4
+
___ Li2CO3

___ ZnCO3
5
15
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?
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 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.212
questions p. 215 2 , 4
(writing and balancing equations)
read p.223 - 226
questions p. 227 1 - 6
(stoichiometry – answers provided)