1. Chapter 8 Chemistry 1L Cypress Creek High School
Unit 8: Ionic Bonds
Chapter 8
Chemistry 1L
Cypress Creek High School

2. Part 4: Percent Composition & Empirical Formula

3. Percent Composition
Percent Composition is the percentage by mass of each element in a compound
The percent composition is found by using the following formula:

4. Percent Composition What is the percent of C & H in C2H6?
79.89% carbon & 20.11% hydrogen
HYDROGEN
CARBON

5. Percent Composition
What is the percent of each element in sodium sulfite, Na2SO3?
2 mole Na x g/mol Na = g Na
1 mole S x g/mol S = g S
3 moles O x g/mol O = g O g
Sodium
% = ( g / g) x 100 = % Na
Sulfur
% = ( g / g) x 100 = % S
Oxygen
% = ( g / g) x 100 = % O
Molar Mass of Na2SO3 =

6. Empirical Formula
An Empirical Formula is the LOWEST whole number ratio of the elements in a compound. The subscript cannot be a fraction.
Ex: The empirical formula for caffeine (C8N4O2H10) is C4N2OH5
C8N4O2H10 is a completely different
chemical than C4N2OH5!
What is the empirical formula
of glucose (C6H12O6)?
CH2O
Back = Carbon
Blue = Nitrogen
Red = Oxygen
Gray = Hydrogen

7. Empirical Formula Calculations
There are 3 types of empirical calculations:
Mole Ratio
Grams
Percent

8. Empirical Formula from Mole Ratios
To calculate the empirical formula from mole ratios, follow these steps:
Divide each mole quantity by the smallest mole quantity
Assign ratios as subscripts
If any of the ratios are not even (ex: 0.5, 1.25, 1.3), multiply them all by the lowest common multiple to achieve whole numbers then place these numbers as the subscripts.

9. Empirical Formula from Mole Ratios
Find the empirical formula for a compound containing 2 mole carbon and 6 mole hydrogen.
C = 2/2 = 1
H = 6/2 = 3
empirical formula = CH3
LOWEST

10. Empirical Formula from Grams
To calculate the empirical formula from grams, follow these steps:
Convert the grams to moles (divide grams by molar mass)
Divide each mole quantity by the smallest mole quantity
Assign ratios as subscripts
If any of the ratios are not even (ex: 0.5, 1.25, 1.3), multiply them all by the lowest common multiple to achieve whole numbers then place these numbers as the subscripts

11. Empirical Formula from Grams
Find the empirical formula of a compound that contains 13.5 grams of calcium, 4.05 grams of carbon, and 16.2 grams of oxygen.
calcium carbon oxygen
13.5 g g g
40.08 g/mol g/mol g/mol
= 0.337
mol
= 0.337
mol
= 1.01
mol
LOWEST
= 1
= 1
= 3
empirical formula = CaCO3

12. Empirical Formula from Percents
To calculate the empirical formula from percent composition, follow these steps:
Change % sign to grams. You assume 100 g of the compound
Convert the grams to moles (divide grams by molar mass)
Divide each mole quantity by the smallest mole quantity
Assign ratios as subscripts
If any of the ratios are not even (ex: 0.5, 1.25, 1.3), multiply them all by the lowest common multiple to achieve whole numbers then place these numbers as the subscripts

13. Empirical Formula from Percents
Calculate the empirical formula of a compound containing 18.8% nickel and 81.2% iodine.
nickel iodine
18.8 g g
58.69 g/mol g/mol
= 0.320
mol
= 0.640
mol
LOWEST
= 1
= 2
empirical formula = NiI2