Download Empirical and molecular formula

EMPIRICAL AND
MOLECULAR FORMULA
DEFINITIONS
Empirical Formula – The lowest whole number
ratio of atoms in a compound. Example: The
empirical formula for the compound C 3 H 12
would be CH 4
Molecular Formula – The actual formula for
the compound- the one that gives the
composition of the molecules that are
present. Example: In the example given above
C 3 H 12 would be the molecular formula.
STEPS FOR DETERMINING THE
EMPIRICAL FORMULA OF A COMPOUND
1. Obtain the mass of each element present (in grams)
2. Determine the number of moles of each type of atom
present (mass  mole conversion)
3. Divide the number of moles of each element by the smallest
number of moles. If you have whole numbers assign them as
subscripts in the formula. If they are not whole numbers go
to step 4.
4. Multiply the numbers from step 3 by the smallest integer
that will convert them to whole numbers. These whole
numbers become the subscripts for the empirical formula
CALCULATING EMPIRICAL FORMULA
EXAMPLE:
Sometimes the masses of each atom in the
element will be given to you like in the
following problem:
A sample of lead arsenate, an insecticide
used against the potato beetle, contains
1.3813 g of lead, 0.00672 of hydrogen,
0.4995 g of arsenic, and 0.4267 g of oxygen.
Calculate the empirical formula for lead
arsenate
CALCULATING EMPIRICAL FORMULA
EXAMPLE:
 Sometimes the percent composition of each element in
the compound will be given to you like the following
example:
 Cisplatin, the common name for a platinum compound
that is used to treat cancerous tumors, has the
composition 65.02% platinum, 9.34% nitrogen, 2.02%
hydrogen, and 26.63% chlorine. Calculate the empirical
formula.
 What steps would I have to take to convert the
percentages of each element in the compound into an
amount in grams?
CALCULATING MOLECULAR FORMULAS
 In order to calculate the molecular formula, two pieces of
information are needed: the empirical formula and the molar
mass of the unknown compound
 Steps for calculating molecular formula:
1.
Calculate the molar mass of the empirical formula
2.
Divide the molar mass of the unknown compound by the molar
mass found in step one
3.
Multiply the whole number you get from step two to the empirical
formula to get the molecular formula. Example: If your molecular
formula is CH 4 and the whole number you get from step 2 is 2 then
your molecular formula is C 2 H 8.
MOLECULAR FORMULA EXAMPLE
 Sometimes you will be given an empirical formula
and a molar mass of an unknown compound like in
the following problem:
 A white powder is analyzed and found to have an
empirical formula of P 2 O 5 . The compound has a
molar mass of 283.88 g. What is the compound’s
molecular formula?
MOLECULAR FORMULA EXAMPLE
 Sometimes you will have to find the empirical formula and you
will be given the molar mass of the unknown compound like in
the following problem:
 A compound used as an additive for gasoline to help prevent
engine knock has the following percent composition: 71 .65 %
Cl, 24.27% C, and 4.07% H. The molar mass is known to be
98.96 g/mol. Determine the empirical formula and the
molecular compound