Empirical Formulas(68)

Chapter 9

Empirical Formulas

Timberlake LecturePLUS

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Types of Formulas The formulas for compounds can be expressed as an empirical formula and as a molecular(true) formula. Empirical

Molecular (true)

CH

C 2H 2

CH

C 6H 6

CO2

CO2

CH2O

Timberlake C 5H 10O5 LecturePLUS

Name

acetylene benzene carbon dioxide ribose

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Empirical Formulas Write your own one-sentence definition for each of the following: Empirical formula

Molecular formula

Timberlake LecturePLUS

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• An empirical formula represents the simplest whole number ratio of the atoms in a compound. • The molecular formula is the true or actual ratio of the atoms in a compound. Timberlake LecturePLUS

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Learning Check EF-1 A. What is the empirical formula for C4H8?

1) C2H4

2) CH2

3) CH

B. What is the empirical formula for C8H14?

1) C4H7

2) C6H12

3) C8H14

C. What is a molecular formula for CH2O?

1) CH2O

2) C2H4O2 Timberlake LecturePLUS

3) C3H6O3 5

Solution EF-1 A. What is the empirical formula for C4H8?

2) CH2 B. What is the empirical formula for C8H14?

1) C4H7 C. What is a molecular formula for CH2O?

1) CH2O

2) C2H4O2

Timberlake LecturePLUS

3) C3H6O3 6

Learning Check EF-2 If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain. 1) SN 2) SN4 3) S4N4

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Solution EF-2 If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain. 3) S4N4 If the actual formula has 4 atoms of N, and S is related 1:1, then there must also 8 be 4 atoms ofTimberlake S. LecturePLUS

Empirical and Molecular Formulas molar mass = simplest mass

a whole number = n

n = 1 molar mass = empirical mass molecular formula = empirical formula n = 2 molar mass = 2 x empirical mass molecular formula = 2 x empirical formula molecular formula = orLecturePLUS > empirical formula9 Timberlake

Empirical Formula

Empirical Mass

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Molecular Formula

Molecular Mass

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Learning Check EF-3 A compound has a formula mass of 176.0 and an empirical formula of C3H4O3. What is the molecular formula?

1) C3H4O3 2) C6H8O6 3) C9H12O9

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Solution EF-3 A compound has a formula mass of 176.0 and an empirical formula of C3H4O3. What is the molecular formula? 2) C6H8O6 C3H4O3 = 88.0 g/EF 176.0 g 88.0

=

2.00 Timberlake LecturePLUS

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Learning Check EF-4 If there are 192.0 g of O in the molecular formula, what is the true formula if the EF is C7H6O4? 1) C7H6O4 2) C14H12O8 3) C21H18O12

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Solution EF-4 If there are 192.0 g of O in the molecular formula, what is the true formula if the EF is C7H6O4? 3) C21H18O12 192 g O

= 3 x O4 or 3 x C7H6O4 Timberlake LecturePLUS

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Finding the Molecular Formula A compound is Cl 71.65%, C 24.27%, and H 4.07%. What are the empirical and molecular formulas? The molar mass is known to be 99.0 g/mol. 1. State mass percents as grams in a 100.00-g sample of the compound. Cl 71.65 g C 24.27 g H 4.07 g Timberlake LecturePLUS

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2. Calculate the number of moles of each element. 71.65 g Cl x 1 mol Cl = 2.02 mol Cl 35.5 g Cl 24.27 g C x

1 mol C 12.0 g C

= 2.02 mol C

4.07 g H x

1 mol H 1.01 g H

=

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4.04 mol H 16

Why moles? Why do you need the number of moles of each element in the compound?

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3. Find the smallest whole number ratio by dividing each mole value by the smallest mole values: Cl: 2.02 = 1 Cl 2.02 C:

2.02 2.02

=

1C

H:

4.04 = 2H 2.02 4. Write the simplest or empirical formula CH2Cl

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5. EM (empirical mass) = 1(C) + 2(H) + 1(Cl) = 49.5 6. n = molar mass/empirical mass Molar mass EM

=

99.0 g/mol 49.5 g/EM

= n=2

7.Molecular formula (CH2Cl)2

= C2H4Cl2

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Learning Check EF-5 Aspirin is 60.0% C, 4.5 % H and 35.5 O. Calculate its simplest formula. In 100 g of aspirin, there are 60.0 g C, 4.5 g H, and 35.5 g O.

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Solution EF-5 60.0 g C x

4.5 g H

___________= ______ mol C

x ___________ = _______mol H

35.5 g O x ___________ = _______mol O Timberlake LecturePLUS

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Solution EF-5 60.0 g C x

1 mol C

=

5.00 mol C

=

4.5 mol H

=

2.22 mol O

12.0 g C 4.5 g H

x

1 mol H 1.01 g H

35.5 g O x

1mol O 16.0 g O

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Divide by the smallest # of moles. 5.00 mol C = ________________ ______ mol O 4.5 mol H = ______ mol O

________________

2.22 mol O = ________________ ______ mol O Are are the results whole numbers?_____ Timberlake LecturePLUS

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Divide by the smallest # of moles. 5.00 mol C = ___2.25__ 2.22 mol O 4.5 mol H 2.22 mol O

=

___2.00__

2.22 mol O = ___1.00__ 2.22 mol O Are are the results whole numbers?_____ Timberlake LecturePLUS

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Finding Subscripts A fraction between 0.1 and 0.9 must not

be rounded. Multiply all results by an integer to give whole numbers for subscripts. (1/2) (1/3) (1/4) (3/4)

0.5 0.333 0.25 0.75

x2 x3 x4 x4

= = = =

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1 1 1 3 25

Multiply everything x 4 C: 2.25 mol C

x 4 = 9 mol C

H: 2.0 mol H

x 4 = 8 mol H

O: 1.00 mol O

x 4 = 4 mol O

Use the whole numbers of mols as the subscripts in the simplest formula

C9H8O4 Timberlake LecturePLUS

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Learning Check EF-6 A compound is 27.4% S, 12.0% N and 60.6 % Cl. If the compound has a molar mass of 351 g/mol, what is the molecular formula?

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Solution EF 6 0.853 mol S /0.853 = 1 S 0.857 mol N /0.853 = 1 N 1.71 mol Cl /0.853 = 2 Cl Empirical formula = SNCl2 = 117.1 g/EF Mol. Mass/ Empirical mass

351/117.1 = 3

Molecular formula = S3N3Cl6 Timberlake LecturePLUS

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