chemistry how compounds form did you know that sand...

9.5 The Laws Governing How Compounds Form

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

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 & YOU


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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.

The Laws of Definite and Multiple Proportions


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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.




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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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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?



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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.


Naming Chemical Compounds


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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 molecular Use 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.

Acid Review the rules

given in Table 9.5.

yes

yes Q = H?

> 2 Elements?

no

no

no

QxRy


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Apply the general formula QxRy 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

yes Q = 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

yes Q = 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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Use the flowchart to write the formula for sodium chromate. The name does not contain prefixes, so the compound is ionic.


Contains prefixes?

Ionic compound Identify the symbols.

Molecular compound Use prefixes to write

the formula.

Polyatomic ions Use Table 9.3 to

obtain the charges.

Roman numerals Give charges for the

cations.

Balance charges Use the crisscross

method.

yes

no

Sodium chromate

Group A elements Use Table 9.1 to

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.


Contains prefixes?



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.


Contains prefixes?



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.

CHEMISTRY & YOU

Contains prefixes?



the formula.


obtain the charges.


cations.


obtain the charges.


method.

yes

no

Name of Compound


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Use the flowchart to name Fe2S3, 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

yes Q = H?

> 2 Elements?

no

no

no

QxRy


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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 Terms

chemistry how compounds form did you know that sand...

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