Percent Composition,
Molecular and Empirical
Formulas
… whew, say that 5 times fast…
Percent Composition
• Have you ever seen CSI?
– You know when the lab people take a
chemical and put it into this “magical
machine” and it give a print out with the name
of the compound… yeah, that machine
doesn’t exist
Percent Composition
• Scientist use percent composition as a
way to identify some compounds
– the percentage by mass of each element in a
compound
– For example, in 1962 scientist made a new
compound that they found was 63.7% Xe and
36.7% F…
• This let them figure out that the formula was XeF4
Percent Composition
• Toluene: C6H5CH3
• What is the percent composition of carbon
in this molecule?
– % carbon =
(mass of carbon)/(mass of molecule)
• Of hydrogen?
Empirical Formula
• Data from the percent composition lets us
figure out the empirical formula for a
compound
– Shows the simplest ratio of a compound.
– Example:
• Molecular formula : H2O2
• Empirical formula : HO
C6H12O6
CH2O
Empirical Formula
• To find the empirical formula:
– calculate the number of moles of each
element in a compound
– Find the ratio of the elements to each other
– If you are given percentages to start (and not
masses), assume a 100g sample
Empirical Formula
• A compound is 38.77g of Cl and 61.23g O.
What is it’s empirical formula
• A compound is made up of 18.0% C,
2.26% H, and 79.7% Cl. What is it’s
empirical formula?
Molecular Formula
• If you know the molecular weight of a
compounds (or the molar mass) and it’s
empirical formula, you can then determine
it’s Molecular formula
– a chemical formula that shows the number
and kinds of atoms in a molecule
– It is always a whole-number ratio
• n(empirical formula) = molecular formula
Molecular Formula
• A compound has an experimental molar
mass of 78g/mol. Its empirical formula is
CH. What is its molecular formula?