1. Percent Composition & Empirical & Molecular Formulas

2. Molecular Formulas The molecular formula
Shows the actual number of each type of atom in a compound
Not always the simplest formula
Ex: C6H12O6 = glucose
Contains 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen in 1 molecule

3. 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.
Glucose
C6H12O6  6 = C1H2O1 = CH2O
molecular formula empirical formula

4. Some Molecular and Empirical Formulas
The molecular formula is the same or a multiple of the empirical.

5. You Try…
1. What is the empirical formula for C4H8? 2. What is the empirical formula for C8H14? 3. Which is a possible molecular formula for CH2O? a) C4H4O4 b) C2H4O2 c) C3H6O3 4. A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain.

6. 3 Steps for determining Chemical Formulas
Determine the percent composition of all elements.
Convert this information into an empirical formula
Find the true number of atoms/ elements in the compound (Molecular Formula)

7. Percent Composition Review
Is the percent by mass of each element in a formula.
Example: Calculate the percent composition of CO2.
CO2 = 1(12.01g/mol) + 2(16.00 g/mol) = g/mol)
12.01 g C x = % C
44.01 g CO2
32.00 g O x = % O
44.01 g CO %

8. You Try…
What is the percent composition of lactic acid, C3H6O3, a compound that appears in the blood after vigorous activity?
The chemical isoamyl acetate C7H14O2 gives the odor of pears. What is the percent carbon in isoamyl acetate?

9. Solution 5
STEP 1: Calculate Molar Mass (Mr) 3C(12.01) + 6H(1.008) + 3O(16.00) = g/mol STEP 2: Calculate % Composition for each element %C = g C x 100 = 40.00% C g %H = g H x 100 = 6.714% H %O = g O x 100 = 53.29% O

10. Solution 6
Molar mass C7H14O2 = 7C(12.01) + 14H(1.008) + 2O(16.00) = g/mol Total C = 7C(12.01) = 84.07g % C = total g C x 100 total g % C = g C x 100 = % C g

11. BRAIN BREAK: The Waiter
Stand up.
Lay a piece of paper or a spiral notebook on the top of your right hand without grabbing it.
While balancing the spiral on your hand, tuck it between your right arm and waist so that the spiral will now be behind you.
You will Flair your arm out away from you for this step.
Again, keep the spiral balanced on top of your hand.
Keep turning your arm so that the spiral will get back to the original spot.
If you have mastered this, try your other hand.

12. Calculating Empirical Formula
If given % composition, assume 100 g of sample
Convert mass of each element to moles (gmol)
Divide each of these numbers by the smallest number
If necessary, multiply by the smallest number possible to make each a whole number
These whole numbers are the subscripts in the empirical formula called mole ratio

13. Step 1: Assume 100 g of Sample 32.4% Na  32.4 g Na 22.5% S  22.5 g S
The percentage composition of a compound is found to be 32.4% sodium, 22.5% sulfur, 45.1 % oxygen. Determine the empirical formula.
Step 1: Assume 100 g of Sample
32.4% Na  g Na
22.5% S  g S
45.1% O  g O
Step 2: Convert grams to moles

14. The percentage composition of a compound is found to be 32
The percentage composition of a compound is found to be 32.4% sodium, 22.5% sulfur, 45.1 % oxygen. Determine the empirical formula.
Step 3: Divide by smallest mol
1.41 mol Na mol S mol O
Step 4: If necessary, multiply by the smallest number possible to make each a whole number
Step 5: Write Empirical Formula
Na2SO4

15. Step 1. Assume 100 g of Sample:
The percentage composition of acetic acid is found to be 39.9% C, 6.7% H, and 53.4% O. Determine the empirical formula of acetic acid.
Step 1. Assume 100 g of Sample:
39.9% C  g C
6.7% H  g H
53.4% O  g O
Step 2. Convert grams to moles

16. The percentage composition of acetic acid is found to be 39. 9% C, 6
The percentage composition of acetic acid is found to be 39.9% C, 6.7% H, and 53.4% O. Determine the empirical formula of acetic acid.
Step 3: Divide by smallest mol
3.32 mol C mol H mol O
Step 4: If necessary, multiply by the smallest number possible to make each a whole number
Step 5: Write Empirical Formula
CH2O

17. BRAIN BREAK: The Crab
Directions: 1. Stand Up 2. Put your arms out in front of you and touch your fingers and thumbs together. 3. Now put lower your middle fingers so that the knuckles touch. Keep them flat against each other. 4. Now un-touch and retouch your thumbs. 5. Now un-touch and retouch your index fingers. 6. Now un-touch and retouch your ring fingers. 7. Lastly, un-touch and retouch your pinkies. Good luck. This one was difficult.

18. 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.

19. Step 1. Assume 100 g of Sample:
Aspirin is 60.0% C, 4.5 % H and 35.5 % O. Calculate its empirical formula.
Step 1. Assume 100 g of Sample:
60.0% C  g C
4.5% H  g H
35.5% O  g O
Step 2. Convert grams to moles

20. Aspirin is 60.0% C, 4.5 % H and 35.5 % O. Calculate its empirical formula.
Step 3: Divide by smallest mol
5.00 mol C mol H mol O
Step 4: If necessary, multiply by the smallest number possible to make each a whole number
Step 5: Write Empirical Formula
C9H8O4