Percent Composition &
Empirical Formula
Percent Composition:
the percentage by mass of each element in a compound.
Empirical Formula:
the formula you would have for a compound if you reduced all of the subscripts
to their lowest terms. E.g., the empirical formula for both C2H2
and C6H6 would be
Determining the Empirical Formula from Percent
Composition Data:
1. Write
the formula as if the subscripts were in grams
2. For
each element in the compound, convert grams to moles.
3. Simplify
the subscripts to simple, whole numbers.
Sample problem: a 10 g
sample of a hydrocarbon is analyzed and found to contain 8.56 g of carbon
and the 1.44 g of hydrogen. What is
the empirical formula of this compound?
1. Write
the formula as C8.56 g H1.44 g
2. Convert
grams to moles:
C: 
H: 
The formula for this compound is therefore C0.713H1.429
3. Convert
the subscripts to simple whole numbers.
The easiest way to do this is to divide them all by the smallest one and
see what happens.
which you can see is
just CH2
If the problem gives percentages instead of actual
mass, just pretend the percentages are out of 100 g total. E.g.,
if you had a compound containing 25.3% nitrogen, you would use 25.3 g of
nitrogen in your calculations.
Rounding: don’t round your fractional subscripts by
more than about 5%. If you have
something like NO2.5, double everything to get N2O5. (This means you need to be able to recognize
decimal equivalents for simple fractions, such as 
, 
, 
, 
, 
, etc.)
Empirical vs. Molecular Formula
If you know the molar mass of the compound, you
can use it to get from the empirical formula to the molecular formula. For example, suppose the molar mass of the
above hydrocarbon was known to be 42.08 g/mol.
The molar mass of the empirical formula (CH2)
would be (1 x 12.011) + (2 x 1.008) = 14.027. Because 42.08 is 3 times as much (14.027 x 3 = 42.08),
the actual formula must have 3 times as much of each atom. This means that the actual formula would be C3H6.