Empirical and Molecular Formulas
Empirical formula determination
The empirical formula is the simpliest ratio of the numbers of atoms of each element that make a
compound. To find the empirical formula of a compound:
 Divide the amount of each element (either in mass or percentage) by its atomic mass. This
calculation gives you moles of atoms for each element that appears in the formula
 Convert the results to small whole number ratios. Often the ratios are obvious. If they are not
divide all of the other quotients by the smallest quotient
Example: Analysis of a certain compound showed that 39.348 grams of it contained 0.883 grams of
hydrogen, 10.497 grams of Carbon, and 27.968 grams of Oxygen. Calculate the empirical
formula of the compound.



First divide the amount by the atomic mass to get the number of moles of each kind
of atom in the formula
Hydrogen
H =
0.883 g
1.01 g mol-1
= 0.874 mol
Carbon
C =
10.497 g
12.01 g mol-1
= 0.874 mol
Oxygen
O =
27.968 g
16.00 g mol-1
= 1.748 mol

Analysis of the ratio s shows that the first tow are identical and that the third is twice the other
two. Therefore the ratio of H to C to O is 1 to 1 to 2. The empirical formula is HCO 2
Molecular formula Determination
To calculate the molecular formula from the empirical formula it is necessary to know the molecular
(molar) mass.
 Add up the atomic masses in the empirical formula to get the factor Divide this number into the
molecular formula mass. If the number does not divide evenly you probably have a mistake in the
empirical formula or its formula mass
 Multiply each subscript in the empirical formula by the factor to get the molecular formula
Example: Suppose the molecular mass of the above compound HCO2 is 90.0. Calculate the molecular
formula.

The empirical formula mass of is
1H
1C
2O

@ 1.0
@ 12.0
@ 16.0
=
=
=
1.0
12.0
32.0
45.0
Note that 45 is exactly half of the molecular mass of 90. So the formula mass of HCO2 is exactly
half of the molecular mass. Hence the molecular formula is double that of the empirical formula
or H2C2O4.
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Worksheet Empirical and Molecular Formula Calculations
1. What is the empirical formula for a compound which contains 0.0134 g of iron,
0.00769 g of sulfur and 0.0115 g of oxygen?
2. Find the empirical formula for a compound which contains 32.8% chromium
and 67.2% chlorine.
3. Find the empirical formula for a compound which contains 0.463 g Tl (#81),
0.0544 g of carbon, 0.00685 g of hydrogen and 0.0725 g oxygen by finding its
empirical formula.
4. What is the empirical formula for a compound which contains 67.1% zinc and
the rest is oxygen?
5. Barry Um has a sample of a compound which weighs 200 grams and contains
only carbon, hydrogen, oxygen and nitrogen. By analysis, he finds that it
contains 97.56 grams of carbon, 4.878 g of hydrogen, 52.03 g of oxygen and
45.53 g of nitrogen. Find its empirical formula.
6. The characteristic odor of pineapple is due to ethyl butyrate, an organic
compound which contains only carbon, hydrogen and oxygen. If a sample of
ethyl butyrate is known to contain 0.62069 g of carbon, 0.103448 g of hydrogen
and 0.275862 g of oxygen, what is the empirical formula for ethyl butyrate?
7. 300 grams of a compound which contains only carbon, hydrogen and oxygen
is analyzed and found to contain the exact same percentage of carbon as it has
oxygen. The percentage of hydrogen is known to be 5.98823%. Find the
empirical formula of the compound.
8. 200.00 grams of an organic compound is known to contain 83.884 grams of
carbon, 10.486 grams of hydrogen, 18.640 grams of oxygen and the rest is
nitrogen. What is the empirical formula of the compound?
9. 300 grams of an organic sample which contains only carbon, hydrogen and
oxygen is analyzed and found to contain 145.946 grams of carbon, 24.3243
grams of hydrogen and the rest is oxygen. What is the empirical formula for the
compound?
10. A certain compound contains 4.0 g of calcium and 7.1 g of chlorine. Is
relative molecular mass is 111. Find its empirical and molecular formulas.
11. A certain compound has 25.9% nitrogen and 74.1% oxygen. Its relative
molecular mass is 108. Find its empirical and molecular formula
12. A certain compound was found to contain 54.0 g of carbon and 10.5 grams
of hydrogen. Its relative molecular mass is 86.0. Find the empirical and the
molecular formulas.
13. A certain compound was found to contain 26.4 g of carbon, 4.4 grams of
hydrogen and 35.2 grams of oxygen. Its relative molecular mass is 60.0. Find
the empirical and the molecular formula.
14. A certain compound was found to contain 78.2 % Boron and 21.8 %
hydrogen. Its relative molecular mass is 27.7. Find the empirical and the
molecular formula.
15. A certain compound contains 7.3%Carbon, 4.5 % hydrogen, 36.4% oxygen,
and 31.8% nitrogen. Its relative molecular mass is 176.0. Find its empirical and
molecular formulas.