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7Chemical Formulas and Composition

Stoichiometry

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Chapter Goals

1. Chemical Formulas

2. Ions and Ionic Compounds

3. Names and Formulas of Some Ionic Compounds

4. Atomic Weights

5. The Mole

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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

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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

Substance consists of two or more elements

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Chemical Formulas

Compound 1 Molecule ContainsHCl 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

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Names and Formulas of Some Ionic Compounds• Tables 1(p221) and 2 (p226) displays the

formulas, charges, and names of some common ions

• Some examples are:– Anions - Cl1-, OH1-, SO4

2-, PO43-

– Cations - Na1+, NH41+, Ca2+, Al3+

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Names and Formulas of 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+ & SO4

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

• Types of ions:

• Simple positive and negative ions are monoatomic in nature. We know their charges from their position on the periodic table.

• What about d block (transition metals) elements that can lose a variable number of electrons?

Chapter 7

• For example copper can lose one or two electrons, producing :

• Cu+ copper I ion (copper one ion)• Cu+2 copper II ion (copper two ion)• So if we know the name of the ionic

compound we will know the charge on the transition metal ion, likewise if we know the formula of the ionic compound we will know the charge on the ion (negative ions always have the same charge)

Chapter 7

• All ionic compounds are electrically neutral-so the + charges must equal the negative.

• EXAMPLES OF THE CRISS-CROSS METHOD • Poly atomic ions: It is possible for a group of

atoms to be bonded together and posses a charge-thus the term polyatomic.

• YOU MUST MEMORIZE THE FOLLOWING IONS!! Their name, formula and charge.

• Nitrate, Sulfate, Phosphate, Carbonate, Bicarbonate, hydroxide , chlorate and permanganate.

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Names and Formulas of Some Ionic Compounds

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

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Names and Formulas of Some Ionic Compounds

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!You do it!

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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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Formula Weights, Molecular Weights, and MolesThe formula weight of calcium nitrate, Ca(NO3)2, is:

You do it!You do it!

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Formula Weights, Molecular 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!You do it!

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Formula Weights, Molecular Weights, and MolesExample 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.

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Formula Weights, Molecular Weights, and MolesExample 2-6: What is the mass of 10.0 billion propane molecules?

You do it!You do it!

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Formula Weights, Molecular 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!You do it!

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

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

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

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Formula Weights, Molecular Weights, and MolesExample 2-8: Calculate the number of O atoms in 26.5 g of Li2CO3.

You do it!You do it!

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Formula Weights, Molecular Weights, and MolesExample 2-9: Calculate the number of mmol in 0.234 g of oxalic acid, (COOH)2.

You do it!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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Percent Composition and Formulas of CompoundsWhat is the percent composition of H in C3H8?

You do it!You do it!

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Percent Composition and 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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Derivation of Formulas from 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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Derivation of Formulas from 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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Derivation of Formulas from 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!You do it!

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Determination of Molecular Formulas

Example 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?