Chemistry
Chapter 11
Empirical and Molecular Formulas
Empirical Formulas
• The simplest whole number ratio of the
elements in a compound (not necessarily
the actual formula for a compound)
• Ex
– H2O2 = HO
– H2O= H2O
– C6H12 = CH2
– C8H16 = CH2
• Can be calculated from % composition
Empirical Formula (Cheat Sheet)
1. % composition (if = 100%) converts to
mass
2. Convert from mass to moles
3. Calculate simplest ratio
(divide all # mol by the smallest # moles)
3 ½. If not all whole numbers, multiply all
by a whole # to create whole #’s
4. Use simplest ratio as subscripts
Examples
• 11.2 % Hydrogen and 88.8% Oxygen
Examples
• 36.84% N and 63.16% O
Examples
• 60% C, 4.44%H, and 35.56% Oxygen
Molecular Formula
• Actual number of moles of each element in
a compound
• Need more info than just % comp
• Need molar mass of the empirical formula
• Need an experimental mass of the actual
compound
Molecular Formula (Cheat Sheet)
1. Use steps to calculate empirical formula
2. Calculate molar mass of empirical
formula
3. The question will give you experimental
mass of the compound
4.
Experimental mass
. =
Empirical form. Molar mass
5. Multiply subscripts in empirical by your
answer (whole number)
Example
• A colorless liquid is found to be composed of
46.68% N and 53.32% O and has a molar
mass of 60.01 g/mol. What is the molecular
formula?