Chapter Three:Chapter Three:Chapter Three:

STOICHIOMETRYSTOICHIOMETRY

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Contents p76

Stoichiometry - The study of quantities of materialsconsumed and produced in chemicalreactions.

3-1 Counting by Weighing

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3-2 Atomic Masses

1.0836129CMassCMass

12

13

amu12.01amu)3.0034(0.0111)(1amu)2(0.9889)(1

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Average atomic mass

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Exact number by definition

amu13.003355amu))(12(1.0836129CofMass 13

Elements occur in nature as mixtures of isotopesCarbon = 98.89% 12C; 1.11% 13C; <0.01% 14C 12.01.

Carbon atomic mass = amu13.0034of1.11%amu12of98.89%

Schematic Diagram of a MassSpectrometer

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Figure 3.1

Ex 3.1 The Average Mass of an ElementP81

When a sample of natural copper is vaporized and injected

into a mass spectrometer, the results shown in Fig. 3.3 are

obtained. Use these data to compute the average mass of

natural copper. (The mass values for 63Cu and 65Cu are

62.93 amu and 64.93 amu, respectively.)Solution:

3-3 The Mole p82

•The number equal to the number ofcarbon atoms in exactly 12 grams ofpure 12C.

•1 mole of anything = 6.022 x 1023

units of that thing.

Ex 3.2 Determining the Mass of a Sampleof Atoms

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Americium is an element that does not occur naturally. It

can be made in very small amounts in a device known as a

particle accelerator. Compute the mass in grams of a

sample of americium containing six atoms.Solution:

Ex 3.4 Calculating Numbers of AtomsP85

A silicon chip used in an integrated circuit of a

microcomputer has a mass of 5.68 mg. How many silicon

(Si) atoms are present in the chip?

Solution:

P85Ex 3.5 Calculating the Numberof Moles and Mass

Cobalt (Co) is a metal that is added to steel to improve its

resistance to corrosion. Calculate both the number of moles

in a sample of cobalt containing 5.00 × 10 20 atoms and the

mass of the sample.Solution:

3.4 Molar Mass

Molar mass: The mass in grams of onemole of the compound.

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Ex 3.6 Calculating Molar Mass P86

Juglone, a dye known for centuries, is produced from the

husks of black walnuts. It is also a natural herbicide (weed

killer) that kills off competitive plants around the black

walnut tree but does not affect grass and other

noncompetitive plants. The formula for juglone is C10H6O3.

(a) Calculate the molar mass of juglone. (b) A sample of

1.56 x 10-2 g of pure juglone was extracted from black

walnut husks. How many moles of juglone does this

sample represent?

Solution: p86

Ex 3.8 Molar Mass and Number of MoleculesP88

Isopentyl acetate (C7H14O2) is the compound responsible

for the scent of bananas. A molecular model of isopentyl

acetate is shown in the margin below. Interestingly, bees

release about 1 μg (1 × 10-6 g) of this compound when

they string. The resulting scent attracts other bees to join

the attack. How many molecules of isopentyl acetate are

released in a typicalbee sting? How many molecules of

isopentyl acetate are released in a typical bee sting? How

many atoms of carbon are present?

Solution: P88

Ex 3.9 Calculating Mass PercentP89

Carvone is a substance that occurs in two forms

having different arrangements of the atoms but

the same molecular formula (C10H14O) and mass.

One type of carvone gives caraway seeds their

characteristic smell, and the other type is

responsible for the smell of spearmint oil. Compute

the mass percent of each element in carvone.

Solution:

3-5 Percent Composition ofCompounds

Mass percent of an element: For iron in

iron (III) oxide, (Fe2O3)

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mass Fe%..

. 11169159 69

100% 69 94%

CH3CH2OH

Now Try This…

Calculate the number of copper atoms in a

63.55 g sample of copper.

React 1

React 2

Consider separate 100.0 gram samples of

each of the following:

H2O, N2O, C3H6O2, CO2

–Rank them from greatest to least

number of oxygen atoms.

3-6 Determining the Formula of aCompound

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Analyzing for Carbon and Hydrogen

H2O

% C % H

p92CO2

Find the formula

CH5N

C2H10N2

C3H15N3

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Formulas

•molecular formula = (empirical formula)n

•[n = integer]

•molecular formula = C6H6 = (CH)6

•empirical formula = CH

P93Ex 3.11 Determining Empiricaland Molecular Formulas

Determine the empirical and molecular formulas for a

compound that gives the following percentages upon

analysis (in mass percents):

71.65％ Cl, 24.27％ C, 4.07％ H. The molar mass is

known to be 98.96 g/mol.

Solution:

Dividing each mole value by 2.021 (the smallestnumber of moles present), we obtain theempirical formula ClCH2.

This substance is composed of moleculeswith the formula Cl2C2H4.

P95Ex 3.13 Determining a Molecular Formula

Caffeine, a stimulant found in coffee, tea , and chocolate,

contains 49.48％ carbon, 5.15％ hydrogen, 28.87％ nitrogen,

and 16.49％ oxygen by mass and has a molar mass of 194.2

g/mol. Determine the molecular formula of caffeine.

Solution:

Rounding the numbers to integers gives themolecular formula for caffeine: C8H10N4O2.

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Molecular Formula DeterminationMethod One

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Method 2p96

3-7 Chemical Equations

Balancing a chemical equation

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productsreactantsOHCOOCH 2224

Chemical Equation

A representation of a chemical reaction:

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g)

reactants products

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The Meaning of a Chemical Equationp98

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g)2 C atoms 2 C atoms

6 H atoms 6 H atoms

7 O atoms 7 O atoms

•The equation is balanced.

•1 mole of ethanol reacts with 3 moles of

oxygen to produce 2 moles of carbon dioxide

and 3 moles of water.

3-8 Balancing Chemical Equations p98

Balancing Chemical Equations

React 3

Which of the following are true concerning balancedchemical equations? There may be more than one truestatement.

–The number of molecules is conserved.

–The coefficients tell you how much of each substanceyou have.

–Atoms are neither created nor destroyed.

–The coefficients indicate the mass ratios of thesubstances used.

–The sum of the coefficients on the reactant sideequals the sum of the coefficients on the product side.

Notice

•The number of atoms of each type of element must be

the same on both sides of a balanced equation.

•Subscripts must not be changed to balance an equation.

•A balanced equation tells us the ratio of the number

of molecules which react and are produced in a

chemical reaction.

•Coefficients can be fractions, although they are

usually given as lowest integer multiples.

•Trial and error is a valid method to balance a chemical

equation.

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Ex 3.14 Balancing a Chemical EquationP100

Chromium compounds exhibit a variety of bright colors.

When solid ammonium dichromate, (NH4)2Cr2O7, a vivid

orange compound, is ignited, a spectacular reaction occurs,

as shown in the two photographs on the next page.

Although the reaction is actually somewhat more complex,

let’s assume that the products are solid chromium (III)

oxide, nitrogen gas (consisting of N2 molecules), and water

vapor. Balance the equation for this reaction.

Solution: p100

3-9 Stoichiometric Calculations:Amounts of Reactions andProducts

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Ex 3.16 Chemical Stoichiometry I p105

Solid lithium hydroxide is used in apace vehicles to remove

exhaled carbon dioxide from the living environment by

forming solid lithium carbonate and liquid water. What mass

of gaseous carbon dioxide can be absorbed by 1.00 kg of

lithium hydroxide?Solution:

3-10 Calculations Involving aLimiting Reagent

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Limiting Reactants

Mixture of CH4 and H2O Molecules

Methane and Water Reacting p107

Notice

We cannot simply add the total moles of all

the reactants to decide which reactant

mixture makes the most product. We must

always think about how much product can be

formed by using what we are given, and the

ratio in the balanced equation.

React 4

The limiting reactant is the reactant

a) for which you have the lowest mass in

grams.

b) that has the lowest coefficient in the

balanced equation.

c) that has the lowest molar mass.

d) that is left over after the reaction has

gone to completion.

e) none of the above.

Ex3.18 Limiting Reactantp110

Nitrogen gas can be prepared by passing gaseous ammonia

over solid copper (II) oxide at high temperature. The other

products of the reaction are solid copper and water vapor. If

a sample containing 18.1 g of NH3 is reacted with 90.4 g of

CuO, which is the limiting reactant? How many grams of N2

will be formed?

Solution:

Ex 3.19 Calculating Percent Yield

Methanol (CH3OH), also called methyl alcohol, is the

simplest alcohol. It is used as a fuel in cars and is a

potential replacement for gasoline. Methanol can be

manufactured by combination of gaseous carbon

monoxide and hydrogen. Suppose 68.5 kg CO(g) is

reacted with 8.60kg H2(g). Calculate the theoretical yield

of methanol. If 3.57 x 104 g CH3OH is actually produced,

what is the percent yield of methanol?

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Solution: p111

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Reaction Between Hydrogen and Oxygen