1. Chapter 9 9.5 The Laws Governing How Compounds Form
Chemical Names and Formulas
9.1 Naming Ions
9.2 Naming and Writing Formulas for
Ionic Compounds
9.3 Naming and Writing Formulas for
Molecular Compounds
9.4 Naming and Writing Formulas for
Acids and Bases
9.5 The Laws Governing How
Compounds Form
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2. Did you know that sand from a beach can be used to make glass?
CHEMISTRY & YOU
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.
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3. The Laws of Definite and Multiple Proportions
How is the law of definite proportions consistent with Dalton’s atomic theory?
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4. The Laws of Definite and Multiple Proportions
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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5. The Laws of Definite and Multiple Proportions
Law of Definite Proportions
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.
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6. The Laws of Definite and Multiple Proportions
Law of Definite Proportions
For example, magnesium sulfide (MgS) is composed of magnesium cations and sulfide anions.
If you could take g of magnesium sulfide and break it down into its elements, you would obtain g of magnesium and g of sulfur.
The Mg:S ratio of these masses is /56.87 or 0.758:1; this ratio never changes.
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7. The Laws of Definite and Multiple Proportions
Law of Definite Proportions
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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8. The Laws of Definite and Multiple Proportions
Law of Definite Proportions
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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9. The Laws of Definite and Multiple Proportions
Law of Multiple Proportions
In the early 1800s, Dalton and others studied pairs of compounds that contain the same elements but have different physical and chemical properties.
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10. The Laws of Definite and Multiple Proportions
Law of Multiple Proportions
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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11. The Laws of Definite and Multiple Proportions
Law of Multiple Proportions
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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12. The Laws of Definite and Multiple Proportions
Law of Multiple Proportions
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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13. The Laws of Definite and Multiple Proportions
Law of Multiple Proportions
Both water and hydrogen peroxide obey the law of definite proportions.
In every sample of hydrogen peroxide, 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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14. The Laws of Definite and Multiple Proportions
Law of Multiple Proportions
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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15. The Laws of Definite and Multiple Proportions
Law of Multiple Proportions
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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16. Calculating Mass Ratios
Sample 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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17. Analyze List the knowns and the unknown. 1
Sample Problem 9.10
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 O Compound B = 6.71 g C and 17.9 g O
UNKNOWN Mass ratio of C per g O in two compounds = ? 1
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18. Solve Solve for the unknown.
Sample Problem 9.10
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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19. Solve Solve for the unknown.
Sample Problem 9.10
Solve Solve for the unknown.
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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20. Solve Solve for the unknown.
Sample Problem 9.10
Solve Solve for the unknown.
Calculate the mass ratio to compare the two compounds. 2
0.748 g C
0.375 g O
1.99 1 = ≈ 2
To calculate the mass ratio, compare the masses of one element per gram of the other element in each compound.
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21. Solve Solve for the unknown.
Sample Problem 9.10
Solve Solve for the unknown.
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 O
1.99 1 = ≈ 2
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22. Evaluate Does this result make sense?
Sample Problem 9.10
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. 3
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23. 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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24. 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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25. 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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26. Practicing Skills: Chemical Names and Formulas
Naming Chemical Compounds
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.
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27. Practicing Skills: Chemical Names and Formulas
Naming Chemical Compounds
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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28. Practicing Skills: Chemical Names and Formulas
Naming Chemical Compounds
Here are some guidelines for helping you name a chemical compound from the chemical formula.
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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29. Practicing Skills: Chemical Names and Formulas
Naming Chemical Compounds
Here are some guidelines for helping you name a chemical compound from the chemical formula.
When a polyatomic ion that includes oxygen is in the formula, the compound name generally ends in -ite or -ate.
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30. Practicing Skills: Chemical Names and Formulas
Naming Chemical Compounds
Here are some guidelines for helping you name a chemical compound from the chemical formula.
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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31. Practicing Skills: Chemical Names and Formulas
Binary molecular
Use prefixes in
the name.
Q = Group A?
Name the ions.
Q = Metal?
yes no
Name the ions; use a Roman numeral with the cation.
Acid
Review the rules given in Table 9.5.
Q = H?
> 2 Elements?
QxRy
This flowchart provides you with a sequence of questions for naming a compound when you know its formula.
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32. Practicing Skills: Chemical Names and Formulas
Binary molecular
Use prefixes in
the name.
Q = Group A?
Name the ions.
Q = Metal?
yes no
Name the ions; use a Roman numeral with the cation.
Acid
Review the rules given in Table 9.5.
Q = H?
> 2 Elements?
QxRy
Apply the general formula QxRy to each compound.
Q and R can be atoms, monatomic ions, or polyatomic ions.
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33. Practicing Skills: Chemical Names and Formulas
Binary molecular
Use prefixes in
the name.
Q = Group A?
Name the ions.
Q = Metal?
yes no
Name the ions; use a Roman numeral with the cation.
Acid
Review the rules given in Table 9.5.
Q = H?
> 2 Elements?
QxRy
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.
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34. Practicing Skills: Chemical Names and Formulas
Writing Chemical Formulas
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.
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35. Practicing Skills: Chemical Names and Formulas
Writing Chemical Formulas
In writing a chemical formula from a chemical name, it is helpful to remember the following guidelines.
An -ite or -ate ending means a polyatomic ion that includes oxygen is in the formula.
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36. Practicing Skills: Chemical Names and Formulas
Writing Chemical Formulas
In writing a chemical formula from a chemical name, it is helpful to remember the following guidelines.
Prefixes in a name generally indicate that the compound is molecular.
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37. Practicing Skills: Chemical Names and Formulas
Writing Chemical Formulas
In writing a chemical formula from a chemical name, it is helpful to remember the following guidelines.
A Roman numeral after the name of a cation shows the ionic charge of the cation.
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38. Practicing Skills: Chemical Names and Formulas
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.
Group A elements
Use Table 9.1 to obtain the charges.
Balance charges
Use the crisscross method.
yes no
Name of Compound
The questions in this flowchart will help you write the formula for a compound when you know its name.
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39. Practicing Skills: Chemical Names and Formulas
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.
Use the flowchart to write the formula for sodium chromate.
The name does not contain prefixes, so the compound is ionic.
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40. Practicing Skills: Chemical Names and Formulas
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.
Group A elements
Use Table 9.1 to obtain the charges.
Balance charges
Use the crisscross method.
yes no
Sodium chromate
The ions are sodium ion and chromate ion.
Sodium is a Group A element, so its ionic charge is +1.
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41. Practicing Skills: Chemical Names and Formulas
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.
Group A elements
Use Table 9.1 to obtain the charges.
Balance charges
Use the crisscross method.
yes no
Sodium chromate
Chromate ion is a polyatomic ion, so use Table 9.3 to obtain its charge (2–).
Balance the charges to obtain the formula Na2CrO4.
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42. Use the flowchart to help you write the formula for silicon dioxide.
CHEMISTRY & YOU
Use the flowchart to help you write the formula for silicon dioxide.
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.
Group A elements
Use Table 9.1 to obtain the charges.
Balance charges
Use the crisscross method.
yes no
Name of Compound
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43. Use the flowchart to help you write the formula for silicon dioxide.
CHEMISTRY & YOU
Use the flowchart to help you write the formula for silicon dioxide.
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.
Group A elements
Use Table 9.1 to obtain the charges.
Balance charges
Use the crisscross 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 SiO2.
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44. Name the ions; use a Roman numeral with the cation.
Use the flowchart to name Fe2S3, which is commonly known as “fool’s gold” because of its shine and color.
Binary molecular
Use prefixes in
the name.
Q = Group A?
Name the ions.
Q = Metal?
yes no
Name the ions; use a Roman numeral with the cation.
Acid
Review the rules given in Table 9.5.
Q = H?
> 2 Elements?
QxRy
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45. Name the ions; use a Roman numeral with the cation.
Use the flowchart to name Fe2S3, which is commonly known as “fool’s gold” because of its shine and color.
Binary molecular
Use prefixes in
the name.
Q = Group A?
Name the ions.
Q = Metal?
yes no
Name the ions; use a Roman numeral with the cation.
Acid
Review the rules given in Table 9.5.
Q = H?
> 2 Elements?
QxRy
FeS is called iron(II) sulfide.
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46. Key Concepts
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.
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47. Key Concepts
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.
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48. Glossary Terms
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
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49. END OF 9.5
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