Percent Composition:
The percent by mass on an element in a compound
Is the number of grams of the element in a formula
divided by the total mass of the formula.
Steps to determine Percent Composition
1. Find the molar mass of the formula
- Ex. H2O H = 2g
O = 16g
total = 18g
2. Divide each element by the total and multiply by 100%
- Ex. 2g = .11 x 100 = 11%
18g
Empirical Formula:
The Empirical Formula of a compound shows the
smallest whole number ratio of the atoms in the compound
Which of the following are empirical formulas?
Sodium peroxide, NaO
Terephthalic acid, C8H6O4
Phenobarbital, C12H12N2O3
Now that you can identify an empirical formula, let us
learn to calculate one.
There are three steps in calculating an empirical formula
Let us try an example
Ex. Calculate the empirical formula of a compound that
contains 67.6% Hg, 10.8% S, 21.6%O
1. Change the percentage to grams and convert the mass
to moles for each element in the formula
67.6g Hg x 1 mole Hg = .34 mol Hg
201g Hg
10.8g S x 1 mole S = .34 mol S
32g S
21.6g O x 1 mole O = 1.4 mol O
16g O
2. Find the lowest molar value and divide it into itself
and all the other molar values
67.6g Hg x 1 mole Hg = .34/.34 = 1
201g Hg
10.8g S x 1 mole S = .34/.34 = 1
32g S
21.6g O x 1 mole O = 1.4/.34 = 4
16g O
3. The number just calculated becomes the subscript of
that atom in the formula.
67.6g Hg x 1 mole Hg = .34/.34 = 1 = Hg1
201g Hg
10.8g S x 1 mole S = .34/.34 = 1 = S1
32g S
21.6g O x 1 mole O = 1.4/.34 = 4 = O4
16g O
Therefore, the empirical formula is HgSO4
Look familiar? Mecuric Sulfate!
So lets review
List the steps for percent composition. Try the sample
problems on pages 306,307
List the steps for empirical formula calculations. Try the
sample problems on page 310
Work on the handouts – if you lost them they are on the
web-page
Now let us learn how to calculate molecular formulas:
-The molecular formula of a compound is either the same
as its empirical formula or a aimple whole number multiple
of the empirical formula.
Ex. CH empirical, C2H4 molecular
Remember an empirical formula can not be reduced but a
molecular formula can be reduced.
Determining Molecular Formulas:
1. Calculate the empirical formula
2. Calculate the molar mass of the empirical formula
3. Divide the mass of the molecular formula by the
molar mass of the empirical formula
4. Use that value to multiply with each subscript of the
empirical formula.
Remember our example of an empirical formula for HgSO4?
Well the sum of this formula is 297g what would the
Molecular formula be if the molecular mass was 594g?
594g/297g = 2
therefore
Hg2(SO4)2 becomes Hg2(SO4)2