1. Percent Composition

2. Imagine having the following amount of money in your pocket: 1 quarter 7 dimes 1 nickel What percentage of each coin do you have as it pertains to their total worth?

3. Percent Composition Calculate the molar mass of the entire compound.
Calculate the molar masses for all of the elements within the compound.
Divide each of the individual element masses (Step 2) by the total molar mass (Step 1) and multiply by 100

4. (2 × 23.0g/mol) + (1 × 32.0g/mol) + (4 × 16.0g/mol)
Percent Composition
Calculate the percent composition for each element present in sodium sulfate, Na2SO4
Step 1:
The molar mass of sodium sulfate is
(2 × 23.0g/mol) + (1 × 32.0g/mol) + (4 × 16.0g/mol)
= 142g/mol

5. Percent Composition
Calculate the percent composition for each element present in sodium sulfate, Na2SO4 Step 2: Of each mole of compound, 2 × 23.0g = 46.0g are sodium 1 × 32.0g = 32.0g are sulfur 4 × 16.0g = 64.0g are oxygen

6. Percent Composition
Calculate the percent composition for each element present in sodium sulfate, Na2SO4 Step 3: Divide each quantity from Step 2 by the molar mass of sodium sulfate (142g) 46 g/142 g x 100 = 32.4% sodium 32g/142g x 100 = 22.5% sulfur 64/142g x 100 = 45.1% oxygen

7. Learning Check
What is the percent of carbon in C5H8NO4 (MSG monosodium glutamate), a compound used to flavor foods and tenderize meats?
1) % C
2) % C
3) % C
Molar mass = g/mole
= g C x 100
146 g MSG

8. Time to go backwards...
How to find the chemical formulas from their percent composition
#1 Assume that you have 100g of the unknown compound.
For example, if you assume that you have 100g of a compound composed of 60.3% magnesium and 39.7% oxygen, you know that you have 60.3g of magnesium and 39.7g of oxygen.

9. Time to go backwards...
#2 Convert the assumed masses from Step 1 into moles by using their molar masses. #3 Identify the lowest mole calculation you found in Step 2. Divide all of the moles you calculated in Step 2 by this lowest mole calculation. This answer gives you the mole ratio (x:y) of the elements of the compound.

10. Time to go backwards...
#4 (If needed) Convert your mole ratios to whole numbers. Multiply each number within the mole ratio by the smallest possible number which produces whole numbers for all the elements.
For example, if your mole ratio is 1:0.5 the chemical formula cannot be N1O0.5
But, if you multiply 1:0.5 by two, you would convert all of the numbers within the mole ratio to whole numbers and end up with N2O

11. Time to go backwards...
#5 Write the chemical formula by attaching these whole-number mole ratios as subscripts to the chemical symbol of each element.
For example, if your mole ratio is 2:1 the chemical formula would be H2O

12. Don’t worry too much if your calculations are a little off when doing these problems. Remember, there is always a little error in all our measurements! If your mole ratio in Step 3 turn out to be 2.3, you can pretty much guess that the answer should be 2.

13. More practice !

14. What’s the empirical formula of a molecule containing 65. 5% carbon, 5
What’s the empirical formula of a molecule containing 65.5% carbon, 5.5% hydrogen, and 29.0% oxygen?
If the molar mass of the compound in problem #1 is 110 grams/mole, what’s the molecular formula?
What’s the empirical formula of a molecule containing 18.7% lithium, 16.3% carbon, and 65.0% oxygen?
If the molar mass of the compound in problem #3 is 73.8 grams/mole, what’s the molecular formula?
A component of protein called serine has an approximate molar mass of 100 g/mole. If the percent composition is as follows, what is the empirical and molecular formula of serine?
C = % H= % O = % N= %

15. C3H3O mass = 55 g/mole
C6H6O2
Li2CO3
C3H7NO3 empirical formula
C3H7NO3 molecular formula