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Empirical Formulas

formaldehyde CH2O(toxic preservative)

acetic acid C2H4O2

(vinegar)

glucose C6H12O6

(simple sugar)

all have the same basic or “empirical” formula

CH2O

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Slide 2 of 40

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Percent Composition and Chemical Formulas

>empirical formula:

the smallest whole-number ratio of the atoms in a compound.

Examples:

CH3 HO PbO2 C2H6N

Non-Examples:

C2H6 H2O2 Pb2O4 C6H18N3

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Slide 3 of 40

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Percent Composition and Chemical Formulas

>

Acetylene (C2H2) is a gas used in welder’s torches.

These two compounds of carbon both have the same empirical formula (CH) but different molecular formulas.

Styrene (C8H8) is used in making polystyrene.

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SAMPLE PROBLEM

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Percent to Mass

Mass to Mole

Divide by Small

Times ‘till Whole

What is the empirical formula of a compound that is analyzed and found to contain 25.9% N and 74.1% O?

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SAMPLE PROBLEM

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What is the empirical formula of a compound that is analyzed and found to contain 25.9% N and 74.1% O?

Percent to Mass

Mass to Mole

Divide by Small

Times ‘till Whole

25.9 g N

74.1 g O

x 1 mol N 14.01 g N

x 1 mol O 16.00 g O

= 1.85 mol N

= 4.63 mol O

________ 1.85

________ 1.85

= 1 N

= 2.5 O

x 2 = 2 N

x 2 = 5 O

N2O5

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Practice Problems

62.1 g C

13.8 g H

24.1 g N

x 1 mol C 12.01 g C

x 1 mol H 1.01 g H

x 1 mol N 14.01 g N

= 5.17 mol C

= 13.66 mol H

= 1.72 mol N

________ 1.72

_________ 1.72

________ 1.72

= 3 C

= 8 H

= 1 N

C3H8N

1,6-diaminohexane is used to make nylon. What is the empirical formula if it is 62.1% C, 13.8% H, and 24.1% N?

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Slide 7 of 40

Percent Composition and Chemical Formulas

>molecular formula:

a whole-number multiple of the empirical formula of a compound

formaldehyde CH2O

acetic acid C2H4O2

glucose C6H12O6

all have the same empirical formula

CH2O

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SAMPLE PROBLEM

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Calculate the molecular formula of a compound with the empirical formula CH4N and a molar mass of 60.0 g/mol.

molecular mass = multiple of emp. form.empirical mass

empirical mass1(C) + 4(H) + 1(N) =

12.01 + 4(1.01) + 14.01 = 30.06 g/mol

60.0 =30.06

1.996 ≈ 2 2 (CH4N) =

C2H8N2

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SAMPLE PROBLEM

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Calculate the molecular formula of benzene with the empirical formula CH and a molar mass of 78 g/mol.

molecular mass = multiple of emp. form.empirical mass

empirical mass1(C) + 1(H) =

12.01 + 1.01 = 13.02 g/mol

78 =13.02

5.991 ≈ 6 6 (CH) = C6H6

1. An empirical formula shows the __________ of atoms in a compound.

A. lowest common number

B. highest whole ratio

C. lowest whole number ratio

D. average ratio

Quick Quiz!

2. Which of the following is NOT an empirical formula?

A. CH3

B. H2N

C. CH

D. C3H6

Quick Quiz.

3. True or False:

A molecular formula can be the same as an empirical formula.

True False

Example:

Formaldehyde has theempirical formula CH2O,which is its molecular formula as well.

Quick Quiz.

Quick Quiz.

4. Determine the molecular formula of a compound that contains 40.0 % C, 6.71 % H, and 53.29 % O by mass and has a molar mass of 60.05 g/mol.

A. C2H4O2

B. CH2O

C. C2H3O

D. C2H4O