1. Determining Chemical Formulas
Define empirical formula
Determine empirical formula
Explain relationship between empirical formula and molecular formula
Determine molecular formula from empirical formula

2. Empirical Formula
Empirical Formula – consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number ratio of the different atoms in the compound
Always assume g sample
Change percents to grams
Calculate moles of each element
Divide moles by smallest mole amount to determine ratio
Multiply to get whole numbers (if necessary)

3. Molecular Formula
Molecular Formula – the actual formula of a molecular compound
Example: C2H4 – ethylene
C3H6 – cyclopropane
Both have a 2H:1C ratio

4. Determining Molecular Formula
Must know formula mass to calculate molecular formula
Divide experimental formula mass by empirical mass
Multiply subscripts by quotient
Example – find molecular formula of a compound with an empirical formula of CH and a formula mass of amu

5. Answer
C –
H – 1.008
Total =
÷ ≈ 6
6(CH) = C6H6

6. Determine the Empirical Formula of Aspirin
What is the empirical formula of aspirin? It has 4.48 % H, % C, and % O by mass.

7. Finding Molecular Formula from Empirical Formula
An unknown compound is found in tree sap. It has been shown to be composed of 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. It was also discovered that 5.00 moles of the material has a mass of 900 grams.
What is the molecular formula of this compound?

8. Homework
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9. Combustion Analysis