1. Section 9.3—Analysis of a Chemical Formula
How can we determine a chemical formula?

2. Percent Composition
How is percent composition found?

3. What is Percent Composition?

4. Example #1
Example:
An 8.5 g sample is composed of carbon and hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of the carbon and hydrogen in the sample?

5. Example #1 If total = 8.5 g and C = 5.5 g - then H = 3.0 g Example:
A 8.5 g sample is composed of carbon and hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of the sample?
If total = g
and C = g -
then H =
3.0 g

6. Let’s Practice #1
A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample?

7. Let’s Practice #1
A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample?
27.5 g 
 27.5 g
100
100

8. Example #2
Percent composition can also be determined from a chemical formula
Example:
Find the percent composition, by mass, of CaCl2

9. Example #2
Percent composition can also be determined from a chemical formula
Example:
Find the percent composition, by mass, of CaCl2 Ca Cl 1 2
40.08 g/mole
35.45 g/mole  =
70.90 g/mole +
g/mole
So for 1 mole:
total = g
Ca = g
Cl =
70.90 g

10. Let’s Practice #2
Find the percent composition, by mass, of NaNO3

11. Find the percent composition, by mass, of NaNO3
Let’s Practice #2
Find the percent composition, by mass, of NaNO3 Na N 1
22.99 g/mole
14.01 g/mole  = +
85.00 g/mole O 3
16.00 g/mole
48.00 g/mole

12. Empirical Formulas

13. What’s an Empirical Formula?
Empirical Formula –Lowest possible ratio of atoms
CH2 is the lowest ratio (and empirical formula) of the molecule C3H6

16. Ratio of Atoms in a Molecule
If the subscripts give the ratio of atoms, then they also give the ratio of moles of atoms
In CaCl2 There is 1 Ca atom to every 2 Cl atoms.
THEREFORE: There is 1 mole of Ca atoms to every 2 moles of Cl atoms!

17. Using Mole Ratio of Atoms in a Molecule
If you know the whole number ratio of the moles for each atom…..
then the subscripts of the chemical formula are known
1 mole C
2 mole H
CH2

18. Example
If given percents, use those percents as grams (as if you assume you have a 100 g sample)—Remember %’s add up to 100! 1 2
Change grams to moles for each atom
Divide all moles by the smallest # of moles. We do this to find the lowest possible whole number ratios of the atoms*** 3 4
Use the ratio as subscripts for writing the chemical formula
Example:
Find the empirical formula if a sample contains Ca and Cl and is 36.1% Ca
***If this step doesn’t give whole numbers – you will need to multiply by a factor that will give you a whole number – you will see this in an example 

19. Example
If given percents, use those percents as grams (as if you assume you have a 100 g sample)—Remember %’s add up to 100! 1 2
Change grams to moles for each atom
Find the lowest possible whole number ratio of the atom (divide all moles by the smallest # of moles) 3 4
Use the ratio as subscripts for writing the chemical formula
Example:
Find the empirical formula if a sample contains Ca and Cl and is 36.1% Ca
36.1 g Ca 1
mol Ca
= _____ mol Ca
0.901
40.08
g Ca
CaCl2
63.9 g Cl 1
mol Cl
= _____ mol Cl
1.80
35.45
g Cl
0.901 mol Ca = 1 mol Ca
0.901
1.80 mol Cl = 2 mol Cl
0.901

20. Find the empirical formula if a sample contains
Let’s Practice #3
Find the empirical formula if a sample contains
40.92 g C, 4.58 g H and 54.5 g O
40.92 g C
= _____ mol C
g C
mol C 1
12.01
3.41
4.58 g H
= _____ mol H
g H
mol H 1
1.01
4.53
54.5 g O
= _____ mol O
g O
mol O 1
16.00
3.41
C3H4O3
3.41 mol C = 1 mol C
3.41
4.53 mol H = 1.33 mol H
3.41
3.41 mol O = 1 mol O
3.41
Multiply the ratio (1 : 1.33 : 1) by 3
to make a whole number ratio
(3 : 4: 3)

21. Molecular Formulas

22. What’s a Molecular Formula?
Empirical Formula –Lowest possible ratio of atoms
Molecular Formula – Actual ratio of atoms in a molecule

23. Empirical versus Molecular Formula
The empirical formula is the lowest possible ratio.
The molecular formula is the actual ratio
A molecule with the empirical formula:
Could have one of the following molecular formulas:
NO2
NO2, N2O4, N4O8…
CH2
CH2, C2H4, C4H8…

24. Find the molecular formula.
Example 1
Find the empirical formula, if not given 2
Find the molar mass of the empirical formula
Find the ratio of the molecular formula’s molar mass (must be given to you) to the empirical formula’s molar mass 3
Multiply the empirical formula’s subscripts by the ratio found in step 3. 4
Example:
Empirical formula = C3H4O3. The molecular formula’s molar mass = g/mole.
Find the molecular formula.

25. Find the molecular formula.
Example 1
Find the empirical formula, if not given 2
Find the molar mass of the empirical formula
Find the ratio of the molecular formula’s molar mass (must be given to you) to the empirical formula’s molar mass 3
Multiply the empirical formula’s subscripts by the ratio found in step 3. 4
Example:
Empirical formula = C3H4O3. The molecular formula’s molar mass = g/mole.
Find the molecular formula.
g/mole = 2
88.07 g/mole C 3 
12.01 g/mole =
36.03 g/mole H 4 
1.01 g/mole =
4.04 g/mole
C3H4O3
 2 O 3 
16.00 g/mole = +
48.00 g/mole
C6H8O6
88.07 g/mole