1. Percent Composition and
Chemical Quantities
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. CFU
What is the percent composition of lactic acid, C3H6O3, a compound that appears in the blood after vigorous activity?

4. Solution #1 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. CFU #2
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 #2 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. CFU #3 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 #3 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. CFU #4
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 #4
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. CFU #5 optional with %Comp
A compound contains 7.31 g Ni and 20.0 g Br. Calculate its empirical (simplest) formula.

14. Solution #5 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. CFU #6
Aspirin is 60.0% C, 4.5 % H and 35.5 % O. Calculate its empirical (simplest) formula.

17. Solution #6 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 #6 (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 #6 (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