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:
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?

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

14. Ratio of Atoms in a Molecule
Subscripts in a chemical formula show the ratio of atoms (or ions) in a molecule
a sample of CaCl2 has 1 calcium ion : 2 chlorine ions
If the subscripts give the ratio of atoms, then they also give the ratio of moles of atoms
a sample of CaCl2 has 1 mole of calcium ions : 2 moles of chlorine ions
We can use the unit “mole” to count things
Atoms and ions can be counted by “moles”

15. Using Mole Ratio of Atoms in a Molecule
Therefore, if the ratio of moles of each atom is found…
then the subscripts of the chemical formula are known
1 mole C
2 mole H
CH2

16. Example #3
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

17. Example #3
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

18. 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)

19. Molecular Formulas

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

21. 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…

22. Find the molecular formula.
Example #4 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.

23. Find the molecular formula.
Example #4 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

24. Hydrate Formulas

25. What’s a Hydrate?
Hydrate – Molecule that has water physically attached to it
It’s not dissolved in water…hydrates can be solid, liquid or gas! The Crystal would look dry! The water is trapped within the crystal!
BaI2  2 H2O
This means there are 2 water molecules attached to each BaI2 molecules

26. Hydrate & anhydride
Hydrate = molecule with water molecules physically attached
anhydride = molecule with water removed
The water can be removed by heating the hydrate
Hydrate anhydride + water
heat
Finding the ratio of anhydride molecules to water molecules gives you the hydrate formula

27. Example #5
Example:
What is the percent mass of water on the following hydrate?
MgSO4•7H2O
Percent Mass of Water = The part of the mass that is water X 100
The Whole mass of the hydrate
= __7(H2O) x 100
MgSO4•7H2O
= (18) x 100
246.38
=51.2%