1. Empirical & Molecular Formulas

2. Percent Composition
Determine the elements present in a compound and their percent by mass.
A 100g sample of a new compound contains 55g of element X and 45g of element Y
Percent by mass can be calculated using
Mass of element X 100 = percent by mass
Mass of compound

3. Percent Composition
The percents of all elements in a compound must equal 100.
The percent by mass of each element in a compound is called the percent composition of a compound

4. Percent Composition from the Chemical Formula
If you already know the chemical formula for a compound you can calculate its percent composition
Calculate the molar mass of each element and compound formula unit
a. Assume sample size is one mole
b. Multiply the molar mass of the element by its
subscript in the chemical formula
Divide the mass of the element by the molar mass of the compound unit and multiply by 100

5. Example What is the percent composition of water?
Hydrogen – 1.01 x 2 = 2.02 g H in water
Oxygen – x 1 = g O in water
Molar mass of water = = g/mol of H2O
% of Hydrogen 2.02 g H x = % H in Water
18.02 g H2O
% of Oxygen g O x 100 = % O in Water

6. General Equation for Calculating % by mass
Mass of element in 1 mol compound X 100 Molar mass of compound

7. Example Problem
Determine the percent composition of Sodium Hydrogen Carbonate (NaHCO3)
Na = 1* g = g Na
H = 1* 1.01 g = 1.01 g H
C = 1* g = g C
O = 3* g = g O
83.98 g/mol NaHCO3

8. Example Problem Continued
Sodium g Na x 100 = 27.4 % Na g NaHCO3 Hydrogen 1.01 g H x 100 = 1.2 % H Carbon g C x 100 = 14.3 % C Oxygen 47.97g O x 100 = 57.1 % O

9. Empirical Formula
The formula with the smallest whole number mole ratio of the elements
May or may not be the same as the molecular formula
If the two formulas are different the molecular formula will always be a simple multiple of the empirical formula
The empirical formula for hydrogen peroxide is HO
The molecular formula for hydrogen peroxide is H2O2

10. Empirical Formula
If the % composition is given you can assume the total mass of the compound is 100 grams and that the percent by mass of each element is equal to the mass of that element in grams
The mass of each element can be converted to moles by dividing the molar mass of the element

11. Example
The percent composition of a sulfur oxide is % S and % O.
40.05 g S = mol S
32.07 g/mol S
59.95 g O = mol O
15.99 g/mol O

12. How does 1. 249 mol of S and 3. 749 mol O transfer into subscripts
How does mol of S and mol O transfer into subscripts? They are not in whole numbers.
Divide the mole values by the value of the element with the smallest number of moles.
1.249 mol S = 1 mol S
1.249
3.749 mol O = 3 mol O

13. The simplest whole number mole ratio of S atoms to O atoms is 1:3.
The empirical formula for the oxide of sulfur is SO3
The calculated mole values may not always be whole numbers
In these cases all the mole values must be multiplied by the smallest factor that will make them whole numbers

14. % to Mass
Mass to Mole
Divide by smallest
Multiply until whole

15. Practice Problems
48. Propane is hydrocarbon, a compound composed only of carbon and hydrogen. It is 81.82% carbon and 18.18% hydrogen. What is the empirical formula?

16. The chemical analysis of aspirin indicates that the molecule is 60
The chemical analysis of aspirin indicates that the molecule is 60.00% carbon, 4.44% hydrogen, and 35.56% oxygen. Determine the empirical formula for aspirin.

17. Molecular Formulas
Two or more substances with distinctly different properties can have the same percent composition and the same empirical formula
The subscripts in an empirical formula indicate the simplest whole-number ratio of moles of the elements in a compound
The simplest ratio does not always indicate the actual number of moles in the compound

18. Determining Molecular Formulas
A molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance
The molar mass must be determined through experimentation and compared with the mass represented by the empirical formula