Chapter 02

1

2Chemical Formulas

and Composition

Stoichiometry

2

Chapter Goals

1. Chemical Formulas

2. Ions and Ionic Compounds

3. Names and Formulas of Some Ionic

Compounds

4. Atomic Weights

5. The Mole

3

Chapter Goals

6. Formula Weights, Molecular Weights, and Moles

7. Percent Composition and Formulas of Compounds

8. Derivation of Formulas from Elemental Composition

9. Determination of Molecular Formulas

10.Some Other Interpretations of Chemical Formulas

11.Purity of Samples

4

Chemical Formulas

• Chemical formula shows the chemical composition of the substance.– ratio of the elements present in the molecule or

compound

• He, Au, Na – monatomic elements

• O2, H2, Cl2 – diatomic elements

• O3, P4, S8 - more complex elements

• H2O, C12H22O11 – compounds

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Chemical Formulas

Compound 1 Molecule Contains

HCl 1 H atom & 1 Cl atom

H2O 2 H atoms & 1 O atom

NH3 1 N atom & 3 H atoms

C3H8 3 C atoms & 8 H atoms

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Ions and Ionic Compounds

• Ions are atoms or groups of atoms that possess an electric charge.

• Two basic types of ions:– Positive ions or cations

• one or more electrons less than neutral

• Na+, Ca2+, Al3+

• NH4+ - polyatomic cation

– Negative ions or anions• one or more electrons more than neutral

• F-, O2-, N3-

• SO42-, PO4

3- - polyatomic anions

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Ions and Ionic Compounds

• Sodium chloride

– table salt is an ionic compound

8

Names and Formulas of

Some Ionic Compounds• Table 2-1 gives names of several

molecular compounds.

– You need to know the names of these compounds

• Some examples are:

– H2SO4 - sulfuric acid

– FeBr2 - iron(II) bromide

– C2H5OH - ethanol

9


Some Ionic Compounds

• Table 2-2 displays the formulas, charges,

and names of some common ions

– You must know the names, formulas, and

charges of the common ions in table 2-3.

• Some examples are:

– Anions - Cl1-, OH1-, SO42-, PO4

3-

– Cations - Na1+, NH41+, Ca2+, Al3+

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Some Ionic Compounds• Formulas of ionic compounds are determined by

the charges of the ions.– Charge on the cations must equal the charge on the

anions.

– The compound must be neutral.

• NaCl sodium chloride (Na1+ & Cl1-)

• KOH potassium hydroxide(K1+ & OH1-)

• CaSO4 calcium sulfate (Ca2+ & SO42-)

• Al(OH)3 aluminum hydroxide (Al3+ & 3 OH1-)

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You do it!

• What is the formula of nitric acid?

– HNO3

• What is the formula of sulfur trioxide?

– SO3

• What is the name of FeBr3?

– iron(III) bromide

12



You do it!

• What is the name of K2SO3?

– potassium sulfite

• What is charge on sulfite ion?

– SO32- is sulfite ion

• What is the formula of ammonium sulfide?

– (NH4)2S

13



You do it!

• What is the charge on ammonium ion?

– NH41+

• What is the formula of aluminum sulfate?

– Al2(SO4)3

• What are the charges on both ions?

– Al3+ and SO42-

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Atomic Weights

• Weighted average of the

masses of the constituent

isotopes of an element.

– Tells us the atomic masses of

every known element.

– Lower number on periodic

table.

• How do we know what the

values of these numbers

are?

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The Mole

• A number of atoms, ions, or molecules

that is large enough to see and handle.

• A mole = number of things

– Just like a dozen = 12 things

– One mole = 6.022 x 1023 things

• Avogadro’s number = 6.022 x 1023

– Symbol for Avogadro’s number is NA.

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The Mole

Example 2-1: Calculate the mass of a single Mg

atom, in grams, to 3 significant figures.

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The Mole

Example 2-2: Calculate the number of atoms in

one-millionth of a gram of Mg to 3 significant

figures.

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The Mole

Example 2-3: How many atoms are contained in

1.67 moles of Mg?

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The Mole

Example 2-4: How many moles of Mg atoms are

present in 73.4 g of Mg?

You do it!

20

Formula Weights, Molecular

Weights, and Moles• How do we calculate the formula weight of

a compound?

– sum the atomic weight of each atom

The formula weight of propane, C3H8, is:

3 C 3 12.01 amu 36.03 amu

8 H 8 1.01 amu 8.08 amu

Forula Weight 44.11 amu

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Weights, and MolesThe formula weight of calcium nitrate, Ca(NO3)2,

is:

You do it!

22


Weights, and Moles• One Mole of Contains

– Cl2 or 70.90g 6.022 x 1023 Cl2 molecules

2(6.022 x 1023 ) Cl atoms

– C3H8

You do it!

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Weights, and MolesExample 2-5: Calculate the number of C3H8

molecules in 74.6 g of propane.

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Weights, and MolesExample 2-6: What is the mass of 10.0 billion

propane molecules?

You do it!

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Weights, and MolesExample 2-7: How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.

You do it!

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Weights, and MolesExample 2-7a: How many moles are contained in 60.0 g of ozone, O3?

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Weights, and MolesExample 2-7b: How many molecules are contained in 60.0 g of ozone, O3?

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Weights, and MolesExample 2-7c: How many oxygen atoms are contained in 60.0 g of ozone, O3?

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Weights, and MolesExample 2-8: Calculate the number of O atoms in

26.5 g of Li2CO3.

You do it!

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Weights, and Moles• Occasionally, we will use millimoles.

– Symbol - mmol

– 1000 mmol = 1 mol

• For example: oxalic acid (COOH)2

– 1 mol = 90.04 g

– 1 mmol = 0.09004 g or 90.04 mg

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Weights, and MolesExample 2-9: Calculate the number of mmol in

0.234 g of oxalic acid, (COOH)2.

You do it!

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Percent Composition and

Formulas of Compounds• % composition = mass of an individual

element in a compound divided by the total

mass of the compound x 100%

Determine the percent composition of C in C3H8.

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Formulas of CompoundsWhat is the percent composition of H in C3H8?

You do it!

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Formulas of CompoundsExample 2-10: Calculate the percent composition

of Fe2(SO4)3 to 3 significant figures.

You do it!

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Derivation of Formulas from

Elemental Composition• Empirical Formula - smallest whole-number ratio of

atoms present in a compound– CH2 is the empirical formula for alkenes

– No alkene exists that has 1 C and 2 H’s

• Molecular Formula - actual numbers of atoms of each element present in a molecule of the compound– Ethene – C2H4

– Pentene – C5H10

• We determine the empirical and molecular formulas of a compound from the percent composition of the compound.– percent composition is determined experimentally

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Elemental Composition• We determine the empirical and molecular formulas of a

compound from the percent composition of the compound.– percent composition is determined experimentally

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Elemental CompositionExample 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?

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Elemental CompositionExample 2-12: A sample of a compound contains

6.541g of Co and 2.368g of O. What is the

empirical formula for this compound?

You do it!

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Determination of Molecular

FormulasExample 2-13: A compound is found to contain 85.63% C and

14.37% H by mass. In another experiment its molar mass is

found to be 56.1 g/mol. What is its molecular formula?

Law of Multiple Proportions

• It is possible for two elements, A and B, to

combine to form more than one

compound.

• The ratios of the masses of element B that

combine with a given mass of element A in

each compound can be expressed by

small whole numbers.

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Law of Multiple Proportions

Example 2-14: Show that the compounds NO2

and N2O5 obey the law of multiple proportions.

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Some Other Interpretations of

Chemical FormulasExample 2-15: What mass of phosphorous is

contained in 45.3 grams of (NH4)3PO4?

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Some Other Interpretations of

Chemical FormulasExample 2-16: What mass of ammonium phosphate,

(NH4)3PO4, would contain 15.0 g of N?

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Purity of Samples

• The percent purity of a sample of a

substance is always represented as

mass of pure substance% purity = 100%

mass of sample

- mass of sample includes impurities

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Purity of Samples

Example 2-18: A bottle of sodium phosphate,

Na3PO4, is 98.3% pure Na3PO4. What are the

masses of Na3PO4 and impurities in 250.0 g of this

sample of Na3PO4?

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Chapter 02