1. Empirical Formulas

2. Empirical Formulas
Empirical Formula: consists of the symbols for the elements combined in a compound, with subscripts of the smallest whole number ratio of the different atoms in the compound.
Example
Sugar has the formula of C6H12O6
Its empirical formula is CH2O

3. Empirical Formulas
What is the empirical formulas for the following compounds?
Na2C2O4
C3H6
K2SO4
B2H6

4. Empirical Formulas
You can calculate the empirical formula of a compound from its percent composition.
Example: You know a compound is % B and % H. What is the compounds empirical formula?

5. Empirical Formulas
Step 1: Assume you have a 100 g sample so the percentages change to grams.
78.10 % B is now g B
21.90 % H is now g H

6. Empirical Formulas
Step 2: Convert the grams to moles using the mole highway.
78.10 g B mol B = mol B
g B
21.90 g H mol H = mol H
g H

7. Empirical Formulas
Step 3: Divide each number by the smallest number to get a ratio. Round to the nearest whole or half number
7.224 is the smallest number
7.224/7.224 = 1 B
21.73/7.224 = 3.01  3 H

8. Empirical Formulas
Step 4: If you have any half numbers, double all the numbers. If not move on.

9. Empirical Formulas
Step 5: The numbers are the ratio of each element. Use them to write the formula.
1 B and 3 H makes BH3

10. Empirical Formulas
Lets say we have a compound that is % K, % Cr and % O.

11. Empirical Formulas
Step 1: Assume you have a 100 g sample so the percentages change to grams.
26.56 % K is now g K
35.41 % Cr is now g Cr
38.03 % O is now g O

12. Empirical Formulas
Step 2: Convert the grams to moles using the mole highway.
26.56 g K mol K = mol K
g K
35.41 g Cr mol Cr = mol Cr
g Cr
38.03 g O mol O = mol O
g O

13. Empirical Formulas
Step 3: Divide each number by the smallest number to get a ratio. Round to the nearest whole or half number
.6793 is the smallest number
.6793/.6793 = 1 K
.6810/.6793 =  1 Cr
2.377/.6793 =  3.5 O

14. Empirical Formulas
Step 4: If you have any half numbers, double all the numbers. If not move on.
1 K x 2 = 2 K
1 Cr x 2 = 2 Cr
3.5 O x 2 = 7 O

15. Empirical Formulas
Step 5: The numbers are the ratio of each element. Use them to write the formula.
2 K, 2 Cr and 7 O makes K2Cr2O7

16. Empirical Formulas Try the following problems:
Find the empirical formula for a compound that is % Na, % S and % O.
Find the empirical formula for a compound that is % P and % O.

17. Empirical Formulas
1. Na2SO4 2. P2O5