Pre AP Chemistry
Chapter 9
“Stoichiometry”
How much can a reaction
produce?
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To this point we are able to predict reactants and
products of chemical reactions to a relatively high
degree of accuracy. These are referred to as qualitative
values.
We will now concentrate on predicting amounts of
substances involved in chemical reactions. These are
referred to as quantitative values.
stoichiometry- the branch of chemistry dealing
with quantitative relations of reactants and
products in chemical reactions.
The coefficients of reactants and products in a
chemical equation represent the number of moles
of substances.
In the equation below, determine the
number of moles and the mass of each
substance represented.
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2HCl  H2 + Cl2
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moles hydrochloric acid = _____moles, mass = _____g
moles of hydrogen gas = _____moles, mass = _____g
moles of chlorine gas
= _____moles, mass = _____g
Applications of Stoichiometry
composition stoichiometry- the mass
relationships of elements in chemical
compounds.
 reaction stoichiometry- the mass
relationships among reactants and products
in chemical reactions.
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Reaction-Stoichiometry Problems
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There are four types of reaction-stoichiometry
problems.
1. mole-mole problems
given: moles of a substance
calculate: moles of a substance
2. mole-mass problems
given: moles of a substance
calculate: mass of a substance
Types of reaction-stoichiometry
problems(continued)
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3. mass-mole problems
given: mass of a substance
calculate: moles of a substance
4. mass-mass problems
given: mass of a substance
calculate: mass of a substance
Mole-Mole Calculations
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Ammonia (NH3) is widely used as a fertilizer and
in many household cleansers. How many moles
of ammonia are produced when 6 moles of
hydrogen gas react with an excess of nitrogen
gas?
3H2 + N2  2NH3
6mol H2 (2mol NH3/3mol H3) = 4mol NH3
Sample Problem
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The decomposition of potassium chlorate
(KClO3) is used as a source of oxygen in the
laboratory. How many moles of potassium
chlorate are needed to produce 15 mol of
oxygen?
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2KClO3  2KCl + 3O2
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Page 301, 1-4
Mole-Mass Calculations
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What mass of MgO is produced when 2.00 mol of
Mg burns in air?
2Mg + O2  2MgO
2mol Mg x 2mol MgO/2mol Mg x 40 g/mol =
80g MgO
Sample Problem
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What mass of oxygen combines with 2.00 mol of
magnesium in the reaction in sample problem 1 ?
2MgO  2Mg + O2
2mol Mg x (1mol O2/2mol Mg) x 32g/mol = 32g
O2
Mass-Mole Calculations
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How many moles of mercury(II) oxide (HgO)
are needed to decompose and produce 125g of
oxygen?
2HgO  2Hg + O2
(125g O2/32g/mol) x (2mol HgO/1mol O2)
= 7.8mol HgO
Sample Problem
How many moles of mercury are produced in
sample problem 1?
 2HgO  2Hg + O2
 (125g O2/32g/mol) x (2mol Hg/1mol O2) =
7.8mol Hg
 Notice: the same coefficients mean the same number of
moles!
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Mass-Mass Calculations
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How many grams of NH4NO3 are required to
produce 33.0 g of N2O by decomposition?
NH4NO3  N2O + 2H2O
(33g N2O/44g/mol) x (1/1) x 80g/mol = 60g
NH4NO3
How many grams of water are produced in the
above problem?
Additional Sample Problems
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1. Determine the mass of carbon dioxide produced by
the decomposition of 50.0 grams of calcium carbonate.
2. Determine the mass of calcium needed to burn in air
to produce 14.0 grams of calcium oxide.
3. 60 grams of magnesium ribbon burn in air.
a. How many moles of magnesium burns?
b. How many moles of magnesium oxide are produced?
c. What mass of magnesium oxide is produced?
More Sample Problems
4. 20 grams of sodium hydroxide reacts with
an excess of hydrochloric acid (HCl). Find the
mass of sodium chloride precipitated from this
reaction.
 5. 56 grams of nitrogen gas reacts with
hydrogen to produce ammonia (NH3). How
many grams of ammonia are produced?
 Assignment: Page 311, 1- 5
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Open Response
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Find the mass of calcium oxide produced from
the decomposition of 750g of calcium
carbonate. Find the number of moles of carbon
dioxide produced also.
 CaCO3 → CaO + CO2
How much does a reaction really produce?
Limiting Reactants and Percent Yield
 limiting
reactant - the reactant that
controls the amount of product formed
in a chemical reaction
 excess reactant - the substance that is
not used up completely in a reaction
Sample Problem
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Some rocket engines use a mixture of hydrazine
(N2H4) and hydrogen peroxide (H2O2) as the
propellant system. The reaction equation is
N2H4(l) + 2 H2O2 (l)  N2 (g) + 4H2O (g)
a. Which is the limiting reactant in the equation
when 0.750 mol of N2H4 reacts with 0.500 mol of
H2O2?
b. How much of the excess reactant, in moles,
remains unchanged?
c. How much of each product, in moles, is formed?
Percent Yield
theoretical yield - the maximum amount of
product that can be produced from a given
amount of reactant
 actual yield - the measured amount of
product obtained from a reaction
 percent yield - the ratio of the actual yield
to the theoretical yield, multiplied by 100%
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Sample Problem
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Methanol can be produced through the reaction
of CO and H2 in the presence of a catalyst
catalyst
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CO(g) + 2H2(g) ------------> CH3OH(l)
If 75.0g of CO reacts to produce 68.4g of
CH3OH, what is the percent yield of CH3OH?
 Assignment:
Page 318, 1-4
Stoichiometry Formulas
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Mole-Mole
coeff unk
 mol unk = mol kn x -------------
coeff kn
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Mole-Mass
coeff unk
 mass unk = mol unk x -------------- x mm unk
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coeff kn
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Stoichiometry Formulas
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Mass-Mole
mass kn
coeff unk
 mol unk = --------------- x -------------
mm kn
coeff kn
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Mass – Mass
mass kn
coeff unk
 mass unk = --------------- x -------------- x mm unk
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mm kn
coeff kn
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Open Response
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Quicklime, CaO, is used by farmers to increase the
pH of soil. It reacts with water according to the
reaction equation
 CaO + H2O → Ca(OH)2
Suppose 100Kg of CaO is spread onto a field.
A. How many moles of CaO are in 100Kg?
B. How many moles of Ca(OH)2 are produced?
C. What mass of Ca(OH)2 is produced?
Support your answers with detailed calculations.