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 – from data
Empirical Formula – Chemical formula determined from lab data. 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
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

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 – Chemical formula determined from lab data. 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!

26. Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the crystal“
either bound to a metal center or have crystallized with the metal
When writing a hydrate, a dot is used to separate the formula compound from the number of water molecules
Examples)
magnesium sulfate heptahydrate
cobalt (II) chloride hexahydrate
tin (II) chloride dihydrate

27. 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

28. Example #5 1 Find the mass of anhydride & water if not given 2
Change mass of anhydride & water to moles 3
Find the ratio of the moles water to moles anhydride 4
Write the hydrate formula
Example:
2.46 g MgSO4 hydrate is heated and 1.20 g MgSO4 anhydride is left. Find the hydrate formula.

29. Hydrate = anhydride + water
Example #5 1
Find the mass of anhydride & water if not given 2
Change mass of anhydride & water to moles 3
Find the ratio of the moles water to moles anhydride 4
Write the hydrate formula
Example:
2.46 g MgSO4 hydrate is heated and 1.20 g MgSO4 anhydride is left. Find the hydrate formula.
Hydrate = anhydride + water
2.46 g = 1.20 g + water
Water = 1.26 g
1.20 g MgSO4 1
mol MgSO4
mol H2O = 7.01
mole MgSO4
= _______ mol MgSO4
120.38
g MgSO4
1.26 g H2O 1
mol H2O
= _______ mol H2O
0.0699
MgSO4  7 H2O
18.02
g H2O

30. Calculating % of Water Determine the mass of the hydrated compound
Determine the mass of JUST the water
Then divide
% H2O = Mass of water x 100
Mass of hydrate
Example)
CuSO4 ⋅ 5H2O CuSO4 + 5H2O
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