1. % Composition & Empirical Formulas
Monday, March 7th, 2016

3. % Composition = the mass % of each element in a compound
Calculating Percent Composition (From Formula) *** Assume 1 mole *** Use Molar Mass For H2O % H = mass H x 100 % O = mass 0 x 100

5. In-Class Practice
Determine the % Composition for: NaBr, Ca3N2, Aluminum Phosphate

6. Empirical Formulas
The empirical formula of a compound is the simplest whole number ratio of the atoms present in the compound.
The empirical formula can be found from the percent composition of the compound.

8. Empirical vs. Molecular Formulas
Shows
Simplest whole number ratio of atoms for a compound
The number and kind of atoms that make up one molecule of a compound
How Determined
Mole ratio elements
Mass ratio
(element to compound)
Ex: Glucose
CH2O
C6H12O6
Empirical = Simplest Ratio of atoms Molecular = Actual Ratio of atoms

9. Steps for Determining the Empirical Formula of a Compound
A gaseous compound containing carbon and hydrogen was analyzed and found to consist of 83.65% carbon by mass. Determine the empirical formula of the compound.
Step 1: Obtain the mass of each element present (in grams).
Assume you have 100 g of the compound.
83.65% C = g C
( – 83.65)
16.35% H = g H

10. Steps for Determining the Empirical Formula of a Compound
Step 2: Determine the number of moles of each type of atom present.

11. Steps for Determining the Empirical Formula of a Compound
Step 3. Divide the number of moles of each element by the smallest number of moles to convert the smallest number to 1. If all of
the numbers so obtained are integers, these are the
subscripts in the empirical formula. If one or more of these numbers are not integers, go on to step 4.

12. Steps for Determining the Empirical Formula of a Compound
Step 4. Multiply the numbers you derived in step 3 by the smallest integer that will convert all of them to whole numbers. This set of whole numbers represents the subscripts in the empirical formula.

13. In-Class Practice Problem
*** Assume grams
*** Use Molar Mass to Calculate Moles
*** Determine simplest Mole Ratio
85.62% C % H

14. Extra Example #2:
What is the Empirical Formula if the % composition is 43.2% K, 39.1% Cl, and some O?
1 mole K
43.2 g K = moles K
=1.0 mole K
39.098g K
1 mole Cl
39.1 g Cl = moles Cl
= 1.0 mole Cl
g Cl
1 mole O
17.7 g O = moles O
= 1.0 mole O
15.999g O
The Empirical Formula is KClO (MW =90.550g/mole)
If MW of the real formula is , what is the actual formula?
(90.550)/(90.550) = 1 KClO x 1 = KClO

15. Extra Example #3
What is the Empirical Formula if the % composition is 40.0% C, 6.7% H, and 53.3% O?
1 mole C
40.0 g C = moles C
=1.0 mole C
12.011g C
1 mole H
6.7 g H = moles H
= 2.0 mole H
1.0079g H
1 mole O
53.3 g O = 3.33 moles O
= 1.0 mole O
15.999g O
The Empirical Formula is CH2O (MW =30.026)
If MW of the real formula is , what is the actual formula?
( )/(30.026) = CH2O x 6 = C6H12O6