9.5 The Laws Governing How Compounds Form 1 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 9 Chemical Names and

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

9.5 The Laws Governing 9.5 The Laws Governing How Compounds FormHow Compounds Form

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Chapter 9Chemical Names and Formulas

9.1 Naming Ions9.2 Naming and Writing Formulas for

Ionic Compounds9.3 Naming and Writing Formulas for

Molecular Compounds9.4 Naming and Writing Formulas for

Acids and Bases

9.5 The Laws Governing How Compounds Form



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Did you know that sand from a beach can be used to make glass?

Sand contains the compound silicone dioxide, which is used in glassmaking.

CHEMISTRY & YOUCHEMISTRY & YOU



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The Laws of Definite and Multiple Proportions

How is the law of definite proportions consistent with Dalton’s atomic theory?

The Laws of Definite and The Laws of Definite and Multiple ProportionsMultiple Proportions



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• The compound calcium carbonate (CaCO3) contains three elements—calcium, carbon, and oxygen—combined in the same proportions in every molecule of CaCO3.

• Two laws—the law of definite proportions and the law of multiple proportions—describe the proportions in which elements combine to form compounds.




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• A chemical formula tells you, by means of subscripts, the ratio of atoms of each element in the compound.

• Ratios of atoms can also be expressed as ratios of masses.


Law of Definite Proportions



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• For example, magnesium sulfide (MgS) is composed of magnesium cations and sulfide anions.

• If you could take 100.00 g of magnesium sulfide and break it down into its elements, you would obtain 43.13 g of magnesium and 56.87 g of sulfur.

• The Mg:S ratio of these masses is 43.13/56.87 or 0.758:1; this ratio never changes.





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• Magnesium sulfide obeys the law of definite proportions, which states that in samples of any chemical compound, the masses of the elements are always in the same proportions.

• The law of definite proportions is consistent with Dalton's atomic theory.





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Dalton postulated that atoms combine in simple whole-number ratios. If the ratio of atoms of each element in a compound is fixed, then it follows that the ratio of their masses is also fixed.





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In the early 1800s, Dalton and others studied pairs of compounds that contain the same elements but have different physical and chemical properties.


Law of Multiple Proportions



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• Using the results from these studies, Dalton stated the law of multiple proportions:

• Whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.





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The figure at right demonstrates the law of multiple proportions.• Copper(I) chloride is

green.• Copper(II) chloride

contains the same elements as copper(I) chloride, but this compound is blue.





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• Two familiar compounds, water (H2O) and hydrogen peroxide (H2O2), are formed by the same two elements.

• Although these compounds are formed by the elements hydrogen and oxygen, they have different physical and chemical properties.

• For example, hydrogen peroxide bleaches the dye in most fabrics, but water does not.





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• Both water and hydrogen peroxide obey the law of definite proportions.

• In every sample of hydrogen peroxide, 16.0 g of oxygen are present for each 1.0 g of hydrogen.

• The mass ratio of oxygen to hydrogen is always 16:1.

• In every sample of water, the mass ratio of oxygen to hydrogen is always 8:1.





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If a sample of hydrogen peroxide has the same mass of hydrogen as a sample of water, the ratio of the mass of oxygen in the two compounds is exactly 2:1.



16 g O (in H2O2 sample that has 1 g H)

8 g O (in H2O sample that has 1 g H)=

16

8=

2

1= 2:1



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Two compounds, X and Y, contain equal masses of element B. The ratio of the masses of A in these compounds is 10:5 or 2:1 (a small, whole-number ratio).





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>> Sample Problem 9.10Sample Problem 9.10

Calculating Mass Ratios

Carbon reacts with oxygen to form two compounds. Compound A contains 2.41 g of carbon for each 3.22 g of oxygen. Compound B contains 6.71 g of carbon for each 17.9 g of oxygen. What is the lowest whole-number mass ratio of carbon that combines with a given mass of oxygen?



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1 Analyze List the knowns and the unknown.

Apply the law of multiple proportions to the two compounds. For each compound, find the grams of carbon that combine with 1.00 g of oxygen. Then find the ratio of the masses of carbon in the two compounds. Confirm that the ratio is the lowest whole-number ratio.

KNOWNS

Compound A = 2.41 g C and 3.22 g OCompound B = 6.71 g C and 17.9 g O

UNKNOWN Mass ratio of C per g O in two compounds = ?



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Solve Solve for the unknown.

First, calculate grams of carbon per gram of oxygen in compound A.

2

2.41 g C

3.22 g O

0.748 g C

1.00 g O=



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Then, calculate grams of carbon per gram of oxygen in compound B.

2

6.71 g C

17.9 g O

0.375 g C

1.00 g O=



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Calculate the mass ratio to compare the two compounds.

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To calculate the mass ratio, compare the masses of one element per gram of the other element in each compound.

0.748 g C

0.375 g O1.99

1= ≈ 2

1



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Express the mass ratio as the lowest whole-number ratio.

• The mass ratio of carbon per gram of oxygen in the two compounds is 2:1.

2

0.748 g C

0.375 g O1.99

1= ≈ 2

1



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Evaluate Does this result make sense?

The ratio is a low whole-number ratio, as expected. For a given mass of oxygen, compound A contains twice the mass of carbon as compound B.

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How does the law of multiple proportions explain the fact that two compounds can contain the same elements but have different chemical properties?



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How does the law of multiple proportions explain the fact that two compounds can contain the same elements but have different chemical properties?

Though two compounds may contain the same elements, the proportions of those elements within each compound differ.



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Practicing Skills: Chemical Names and Formulas

What general guidelines can help you write the name and formula of a chemical compound?

Practicing Skills: Chemical Practicing Skills: Chemical Names and FormulasNames and Formulas



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• One of the skills you learned in this chapter is to name chemical compounds.

• You may feel overwhelmed and find it difficult to know when you should or should not use prefixes and Roman numerals in a name.

• Or you may have trouble determining if a compound's name should end in -ate, -ide, or -ite.


Naming Chemical Compounds



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Here are some guidelines for helping you name a chemical compound from the chemical formula.

Follow the rules for naming acids when H is the first element in the formula.





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If the compound is binary, generally the name ends with the suffix -ide. If the compound is a molecular binary compound, use prefixes to indicate the number of atoms.





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When a polyatomic ion that includes oxygen is in the formula, the compound name generally ends in -ite or -ate.





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If the compound contains a metallic cation that can have different ionic charges, use a Roman numeral to indicate the numerical value of the ionic charge in the compound.





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This flowchart provides you with a sequence of questions for naming a compound when you know its formula.


Binary molecularUse prefixes in

the name.

Q = Group A?

Name the ions.

Q = Metal?

Name the ions.

yes

yes yes

no

no

Name the ions; use a Roman numeral with

the cation.

Q = Group A?


the cation.

AcidReview the rules

given in Table 9.5.

yes

yesQ = H?

> 2 Elements?

no

no

no

QxRy



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Apply the general formula Q

xR

y to each

compound.

Q and R can be atoms, monatomic ions, or polyatomic ions.



the name.

Q = Group A?

Name the ions.

Q = Metal?

Name the ions.

yes

yes yes

no

no


the cation.

Q = Group A?


the cation.


given in Table 9.5.

yes

yesQ = H?

> 2 Elements?

no

no

no

QxRy



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For example, to name HNO3, let H = Q and NO3 = R.

Follow the first arrow down to the question “Q = H?”

The answer is yes, so the compound is an acid.

Follow the rules for naming acids.



the name.

Q = Group A?

Name the ions.

Q = Metal?

Name the ions.

yes

yes yes

no

no


the cation.

Q = Group A?


the cation.


given in Table 9.5.

yes

yesQ = H?

> 2 Elements?

no

no

no

QxRy



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In writing a chemical formula from a chemical name, it is helpful to remember the following guidelines.

An -ide ending generally indicates a binary compound.


Writing Chemical Formulas



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An -ite or -ate ending means a polyatomic ion that includes oxygen is in the formula.





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Prefixes in a name generally indicate that the compound is molecular.





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A Roman numeral after the name of a cation shows the ionic charge of the cation.





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The questions in this flowchart will help you write the formula for a compound when you know its name.


Containsprefixes?

Ionic compoundIdentify the symbols.

Molecular compoundUse prefixes to write

the formula.

Polyatomic ionsUse Table 9.3 to

obtain the charges.

Roman numeralsGive charges for the

cations.

Group A elementsUse Table 9.1 to

obtain the charges.

Balance chargesUse the crisscross

method.

yes

no

Name of Compound



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Use the flowchart to write the formula for sodium chromate.

The name does not contain prefixes, so the compound is ionic.


Containsprefixes?



the formula.


obtain the charges.


cations.


method.

yes

no

Sodium chromate


obtain the charges.



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>>The ions are sodium ion and chromate ion.

Sodium is a Group A element, so its ionic charge is +1.


Containsprefixes?



the formula.


obtain the charges.


cations.


obtain the charges.


method.

yes

no

Sodium chromate



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>>Chromate ion is a polyatomic ion, so use Table 9.3 to obtain its charge (2–).

Balance the charges to obtain the formula Na2CrO4.


Containsprefixes?



the formula.


obtain the charges.


cations.


obtain the charges.


method.

yes

no

Sodium chromate



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>>Use the flowchart to help you write the formula for silicon dioxide.


Containsprefixes?



the formula.


obtain the charges.


cations.


obtain the charges.


method.

yes

no

Name of Compound



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>>Use the flowchart to help you write the formula for silicon dioxide.


Containsprefixes?



the formula.


obtain the charges.


cations.


obtain the charges.


method.

yes

no

Name of Compound

The name silicon dioxide contains a prefix, so it is a molecular compound. Using the prefixes, the formula is SiO

2.



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>>Use the flowchart to name Fe

2S

3, which is

commonly known as “fool’s gold” because of its shine and color.


the name.

Q = Group A?

Name the ions.

Q = Metal?

Name the ions.

yes

yes yes

no

no


the cation.

Q = Group A?


the cation.


given in Table 9.5.

yes

yesQ = H?

> 2 Elements?

no

no

no

QxRy



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>>Use the flowchart to name Fe

2S

3, which is

commonly known as “fool’s gold” because of its shine and color.


the name.

Q = Group A?

Name the ions.

Q = Metal?

Name the ions.

yes

yes yes

no

no


the cation.

Q = Group A?


the cation.


given in Table 9.5.

yes

yesQ = H?

> 2 Elements?

no

no

no

QxRy

FeS is called iron(II) sulfide.



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>>If the ratio of atoms of each element in a compound is fixed, then the ratio of their masses is also fixed.

Follow the rules for naming acids when H is the first element. If the compound is binary, generally the name ends with -ide. For a molecular binary compound, use prefixes to indicate the number of atoms. When a polyatomic ion with oxygen is in the formula, the compound name ends in -ite or -ate. If the compound contains a metallic cation that can have different ionic charges, use a Roman numeral to indicate the ionic charge.

Key ConceptsKey Concepts



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An -ide ending usually indicates a binary compound. An -ite or -ate ending indicates a polyatomic ion with oxygen. Prefixes usually indicate a molecular compound. A Roman numeral after the name of a cation shows the ionic charge of the cation.

Key ConceptsKey Concepts



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• law of definite proportions: in samples of any chemical compound, the masses of the elements are always in the same proportion

• law of multiple proportions: whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers

Glossary TermsGlossary Terms



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END OF 9.5END OF 9.5

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9.5 The Laws Governing How Compounds Form 1 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 9 Chemical Names and