1. Unit 6: Chemical Quantities
Chapter 10: Empirical and Molecular Formulas

2. Empirical Formulas Empirical Formulas
The lowest whole-number ratio of atoms of the elements in a compound.
Empirical Formulas are much like creating a recipe of elements.
Orange Juice  1 can of concentrate to 3 cans of water = (OJ)1(H2O)3 = 1:3 ratio
Double Batch
(OJ)2(H2O)6 = 2:6 = 1:3 ratio

3. Empirical Formulas Steps in determining Empirical Formulas :
Assume that the percent given is equal to the number of grams in a 100 gram sample.
Meaning…remove the % and add g for the units.
Convert the grams to moles using the molar mass.
Divide each mole value by the smallest mole calculated to determine a ratio.
If all the values are not near a whole number (_.8-_.2 range), multiply ALL the values by 2 or 3 until the values are all whole numbers.
Give the Empirical Formula

4. Empirical Formulas Practice with Steps
A compound is analyzed and found to contain % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound.
Step 1: Assume percents are grams
Nitrogen = 23.9% = 23.9g
Oxygen = 74.1% = 74.1g

5. Empirical Formulas Practice with Steps
A compound is analyzed and found to contain % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound.
Step 2: Convert to moles using molar mass
Nitrogen = 23.9 g 1 mol =1.85 mol Nitrogen
14 g
Oxygen = 74.1 g 1 mol = 4.63 mol Oxygen
16 g

6. Empirical Formulas Practice with Steps
A compound is analyzed and found to contain % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound.
Step 3: Divide each mole by the smallest mole value
Nitrogen = 1.85 mol ÷ 1.85 mol = 1mol of N
Oxygen = 4.63 mol ÷ 1.85 mol = 2.5 mol of O

7. Empirical Formulas Practice with Steps
A compound is analyzed and found to contain % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound.
Step 4: Multiply the moles by a coefficient to have whole number ratios.
Nitrogen = 1 mol x 2 = 2 mol of N
Oxygen = 2.5 mol x 2 = 5 mol of O
Step 5: Give Empirical Formula
N2O5

8. Molecular Formulas Molecular Formulas Requires two things
Based on Empirical Formulas
Most molecular formulas are empirical formulas, but some molecular formulas are not the lowest ratios.
Example: Hydrogen Peroxide
Empirical Formula  HO
Molecular Formula  H2O2
Requires two things
Mass of the Compound
Empirical Formula of the Compound

9. Molecular Formulas Practice
Calculate the molecular formula of a compound whose mass is 60.0 g and empirical formula is CH4N.
Find the molar mass of the empirical formula
CH4 N = 30.0 g/mol

10. Molecular Formulas Practice
Calculate the molecular formula of a compound whose mass is 60.0 g and empirical formula is CH4N.
Divide the molecular mass by the empirical mass
Molecular mass/ empirical mass
(60.0 / 30.0 ) = 2

11. Molecular Formulas Practice
Calculate the molecular formula of a compound whose mass is 60.0 g and empirical formula is CH4N.
Multiply the empirical formula subscripts by the coefficient found in step 2.
C (1x2) H (4x2) N (1x2) = C2H8N2