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Chapter 4
Quantities of Reactants and
Products
Antoine Lavoisier
1743-1794. “Father
of modern chemistry.”
Recognized true elements.
Used quantitative measurements
in chemical reactions.
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Chemical Equations
• Lavoisier: mass is conserved in a chemical reaction.
• Chemical equations: descriptions of chemical
reactions.
• Two parts to an equation: reactants and products:
2H2 + O2
2H2O
Reactants
Product
2
H2
O2
H2O
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• Stoichiometric coefficients: numbers in front of the
chemical formulas give numbers of molecules or
atoms reacting (and numbers being produced).
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Law of Conservation of Mass:
All reactions must be balanced
CH4 + O2
CO2 + H2O
is not balanced. (Why?)
Count atoms:
Reactants:
1C
4H
2O
Products:
1C
2H
3O
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Balance reactions only by changing coefficients,
not by altering chemical formula
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Combustion is the burning of a substance in oxygen:
C3H8(g) + 5O2(g)  3CO2 (g) + 4H2O(l)
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Which is correct? (Blue=A; Red=B)
a) A2 + B
A2B
b) A2 + 4B
2 AB2
c) 2A + B4
2 AB2
d) A + B2
AB2
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Atomic and Molecular Weights
Percentage Composition from Formulas
Percent composition is the atomic weight for each
element divided by the formula weight of the compound
multiplied by 100:

Atoms of Element AW 
% Element 
 100
FW of Compound
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Percentage Composition from Formulas
What is % O in H2SO4 (by mass)?
FW= (2x1) + (1x32)+ (4x16)= 98 amu
FW of O in H2SO4 = 4 x 16 = 64 amu
%O = 64 x 100 = 65.3%
98
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The Mole*
*MSJ Ch 3 pp 100-104
•The “amu” is an “atomic mass unit.”
•O has a mass of 16 amu – but we can’t weigh out anything
in amu
•If we want to keep the number “16” for the mass of
oxygen in some real units (like grams) then we are dealing
with a whole bunch of atoms (in 16 g of oxygen).
•That bunch of atoms is called a mole.
•Experimentally,
1 mole = 6.02 x 1023 things (atoms)
•This number is called Avogadro’s number.
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The mole is defined so that one mole of a substance has a mass
equal to its AW or MW in grams
Basically, you are replacing amu with grams,
e.g.
The mass of a P atom is 31 amu.
The molar mass of P is 31 grams.
The mass of a Ca atom is 40 amu
The molar mass of Ca is 40 grams
This amount of P or Ca each contains Avogadro’s
number (6.02 x 1023) of atoms of P and Ca.
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The Mole
Experimentally, 1 mole of 12C has a mass of
exactly 12 g. (recall from Ch. 2)
Molar Mass
Molar mass: mass in grams of 1 mole of substance
Units: g/mol or g.mol-1.
Mass of 1 mole of 12C = 12 g exactly
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This photograph shows
one mole of :
solid NaCl (58.5 g),
liquid H2O (18 g), and
gaseous N2 (28 g).
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The Mole
Molar Mass
Molar mass: sum of the molar masses of the atoms:
What is Molar Mass of H2SO4?
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The Mole
Interconverting Masses, Moles, and Numbers of
Particles
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Example 1:
(a) contains
(b) contains
5.00 g of P
mol of P
atoms of P
Example 2: 5.00 x 1024 atoms of C
(a) equals
mol of C
(b) has mass equal to
grams.
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Example 3: 3.5 mol CO2:
(a) has what mass?
(b) contains how many molecules of CO2?
(c) contains how many atoms of O?
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Empirical Formulas from Analyses
Start with mass % of elements (i.e. empirical data) and
calculate a formula.
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Empirical Formulas from Analyses*
Example: compound of N and O
*MSJ Ch 3 pp 104-108
Given analysis: N: 25.9%; O: 74.1%
Assume 100g; N: 25.9 g; O: 74.1 g
Change to mol:
N: 25.9 g x mol = 1.85 mol
14 g
O: 74.1 g x mol = 4.63 mol
16 g
Preliminary emp. Formula: N1.85O4.63
Clean it up: divide both by 1.85:
Get N1O2.5; get rid of fractions, multiply both by 2:
Get N2O5 which is the empirical (simplest) formula
What are some possible molecular formulas?
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Empirical Formulas from Analyses
Molecular Formula from Empirical Formula
Once we know the empirical formula, we need the MW to
find the molecular formula.
Subscripts in the molecular formula are always wholenumber multiples of subscripts in the empirical formula.
Example: suppose compound of C and H has
empirical formula of C3H8 and a MW = 176 g/mol.
What is the molecular formula?
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Quantitative Information from
Balanced Equations
Balanced chemical equation gives number of molecules
(or moles) that react to form products.
Interpretation: balanced equation gives us the ratio of
number of moles of reactant to product (or v.v.)
.These ratios are called stoichiometric ratios.
Example:
2 H2 + O2
Molecules:
2
1
2
1
Moles:
Ratio of O2:H2O = 1:2
2 H 2O
2
2
(either molecules or moles)
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Quantitative Information from
Balanced Equations
The ratio of
grams of
reactant
cannot be
directly
related to the
grams of
product.
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Stoichiometry
Problem:
aluminum sulfide + water
aluminum hydroxide + hydrogen sulfide
(a) Write balanced reaction:
(b) How many g aluminum hydroxide obtained from 10.5 g of
aluminum sulfide?
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Stoichiometry
Problem:
2 NaN3(s)
2Na(s) + 3 N2(g)
(a) how many mol N2 produced from 2.50 mol NaN3?
(b) how many g NaN3 needed to form 6.00 g N2
(c) how many g NaN3 needed to produce 10.0 ft3 of N2?
(1.00 ft3 = 28.3 L; density of N2 = 1.25 g/L)
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Limiting Reactants
If the reactants are not present in stoichiometric
amounts, at end of reaction some reactants are still
present (in excess).
Limiting Reactant: one reactant that is consumed.
O2
H2
O2 INXS
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RECIPE:
3 cups flour + 4 eggs + 2 cups sugar cake
-------------------------------------------------------On Hand:
cups flour
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eggs
20
cups sugar
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How many cakes can be made from these amounts?
What is the “limiting reactant” (LR)
What’s left over and how much of it is left over?
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RECIPE:
3 cups flour + 4 eggs + 2 cups sugar cake
-------------------------------------------------------On Hand:
cups flour
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eggs
28
cups sugar
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How many cakes can be made from these amounts?
What is the “limiting reactant” (LR)
What’s left over and how much of it is left over?
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Limiting Reagent
(a)Assume a reactant (any one) is LR.
Calculate stoichiometric amount of product (any
product) formed.
(b) Pick another reactant and make it the LR.
Calculate the stoichiometric amount of same
product formed.
(c) Whichever reactant gives the smaller
amount of product is the Limiting Reagent
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Limiting Reagent
The LR is always used up in a
chemical reaction. Everything else
is in excess (INXS).
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Limiting Reagent
Problem:
4 NH3 + 5 O2
4 NO + 6 H2O
2.25 g NH3 mixed with 3.75 g O2 and allowed to react
(a) which is LR?
(b) how many grams NO formed?
(c) how much of excess reactant remains?
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Limiting Reactants
Theoretical Yields
The amount of product predicted from stoichiometry
taking into account limiting reagents is called the
theoretical yield.
The percent yield relates the actual yield (amount of
material recovered in the laboratory) to the
theoretical yield:
Actual yield
% Yield 
 100
Theoretical yield
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Theoretical Yield
Problem:
C6H6 + Br2
C6H5Br + HBr
(a) theoretical yield of C6H5Br when 30.0 g
of C6H6 reacts with 65.0 g of Br2?
(b) if actual yield of C6H5Br is 56.7 g,
calculate %yield
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