Determining Chemical Formulas

1. Define empirical formula2. Determine empirical formula3. Explain relationship between

empirical formula and molecular formula

4. Determine molecular formula from empirical formula

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 100.0 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)

Molecular Formula

Molecular Formula – the actual formula of a molecular compound

Example: C2H4 – ethylene

C3H6 – cyclopropane

Both have a 2H:1C ratio

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 78.110 amu

Answer

C – 12.011

H – 1.008

Total = 13.019

78.110 ÷ 13.019 ≈ 6

6(CH) = C6H6

Determine the Empirical Formula of Aspirin

What is the empirical formula of aspirin? It has 4.48 % H, 60.00 % C, and 35.52 % O by mass.

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?

Homework

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Page 237 35-39

Combustion Analysis