1. Percent Composition and
Empirical Formulas
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

2. Percent Composition Percent composition
Is the percent by mass of each element in a formula.
Example: Calculate the percent composition of CO2.
CO2 = 1 C(12.01g) + 2 O(16.00 g) = g/mol)
12.01 g C x = % C
44.01 g CO2
32.00 g O x = % O
44.01 g CO %

3. Learning Check
What is the percent composition of lactic acid, C3H6O3, a compound that appears in the blood after vigorous activity?

4. Solution STEP 1 3C(12.01) + 6H(1.008) + 3O(16.00) = 90.08 g/mol
36.03 g C g H g O
STEP 2
%C = g C x 100 = 40.00% C
90.08 g
%H = g H x 100 = 6.714% H
%O = g O x 100 = % O

5. Learning Check
The chemical isoamyl acetate C7H14O2 gives the odor of pears. What is the percent carbon in isoamyl acetate?
1) %C
2) %C
3) %C
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

6. Solution 3) 64.58 %C Molar mass C7H14O2 = 7C(12.01) + 14H(1.008)
+ 2O(16.00) = g/mol
Total C = 7C(12.01) = g
% C = total g C x 100
total g
% C = g C x 100 = % C g

7. Empirical Formulas The empirical formula
Is the simplest whole number ratio of the atoms.
Is calculated by dividing the subscripts in the actual (molecular) formula by a whole number to give the lowest ratio.
C5H10O5  5 = C1H2O1 = CH2O
actual (molecular) empirical formula
formula

8. Some Molecular and Empirical Formulas
The molecular formula is the same or a multiple of the empirical.
Table 6.3
Copyright © by Pearson Education, Inc. Publishing as Benjamin Cummings

9. Learning Check 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. Which is a possible molecular formula for CH2O?
1) C4H4O4 2) C2H4O ) C3H6O3

10. Solution A. What is the empirical formula for C4H8? 2) CH2 C4H8  4
B. What is the empirical formula for C8H14?
1) C4H7 C8H14  2
C. Which is a possible molecular formula for CH2O?
2) C2H4O ) C3H6O3

11. Learning Check
A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain.
1) SN
2) SN4
3) S4N4

12. Solution
A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain.
3) S4N4
In this molecular formula 4 atoms of N and 4 atoms of S and N are related 1:1. Thus, it has an empirical formula of SN.

13. Learning Check
A compound contains 7.31 g Ni and 20.0 g Br. Calculate its empirical (simplest) formula.

14. Solution Convert 7.31 g Ni and 20.0 g Br to moles.
7.31 g Ni x 1 mol Ni = mol Ni
58.69 g Ni
20.0 g Br x 1 mol Br = mol Br
79.90 g Br
Divide by smallest:
0.125 mol Ni = 1 Ni mol Br = 2 Br
Write ratio as subscripts: NiBr2

15. Converting Decimals to Whole Numbers
When the number of moles for an element is a
decimal, all the moles are multiplied by a small
integer to obtain whole number.
Table 6.4
Copyright © by Pearson Education, Inc. Publishing as Benjamin Cummings

16. Learning Check
Aspirin is 60.0% C, 4.5 % H and 35.5 % O. Calculate its empirical (simplest) formula.

17. Solution STEP 1. Calculate the moles of each element in 100 g.
100 g aspirin contains 60.0% C or 60.0 g C, 4.5% H or 4.5 g H, and 35.5% O or 35.5 g O.
60.0 g C x 1 mol C = mol C
12.01 g C
4.5 g H x 1 mol H = mol H
1.008 g H
35.5 g O x 1mol O = mol O
16.00 g O

18. Solution (continued) STEP 2. Divide by the smallest number of mol.
5.00 mol C = mol C (decimal)
2.22
4.5 mol H = 2.0 mol H
2.22 mol O = mole O

19. Solution (continued)
3. Use the lowest whole number ratio as subscripts
When the moles are not whole numbers, multiply by a factor to give whole numbers, in this case x 4.
C: mol C x 4 = 9 mol C
H: 2.0 mol H x 4 = 8 mol H
O: mol O x 4 = 4 mol O
Using these whole numbers as subscripts the simplest formula is
C9H8O4