1. Unit 9: Covalent Bonding
Chapters 8 & 9
Chemistry 1L
Cypress Creek High School

2. 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:

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

4. 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 =

5. 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

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

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. Molecular Formulas
For many compounds, the empirical formula is not the true formula.
A molecular formula tells the exact number of atoms of each element in a molecule or formula unit of a compound.
The actual formula
The molecular formula for a compound is always a whole-number multiple of the empirical formula.

14. Determining Molecular Formulas
Determine the empirical formula
Find the empirical formula mass (you are given the molecular formula mass)
Divide the molecular formula mass by the empirical formula mass. This is the scalar (multiplier).
Multiply the subscripts in the empirical formula by the multiplier.

15. Calculating Molecular Formula
A compound has an empirical formula of Sc2O3. The molecular mass is 414 g/mol. What is the molecular formula?
Empirical Formula = Sc2O3
Empirical Formula Mass =
2 mol Sc x g/mol Sc = g Sc
3 mol O x g/mol O = g O
molar mass = g Sc2O3
Find the multiplier…
414 g / g = 3 (the multiplier is 3)
Molecular Formula = Sc6O9

16. Practice Molecular Formula
A molecule has 85.6% carbon and 14.5% hydrogen. If the molecule has a mass of 42.1 grams, what is the molecular formula?

17. Be Prepared for Unit 9 Test.
THE END.
Be Prepared for Unit 9 Test.