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Determining Empirical Formulas
Empirical formulas show the relative number of atoms of each element in any sample of a compound. The
smallest possible whole numbers are used for the subscripts.
Steps to determine an empirical formula of a compound:
A. From mass percent
1.
Assume a 100 g sample – use the percent numbers as “grams”.
2.
Convert the mass of each element into moles of the element, using a “mass-to-mole”
conversion.
3.
Divide the resulting number of moles of each element by the smallest number of moles present,
to give the ratio of moles in the compound. Express the ratio as the smallest whole number
ratio. ( “mole ratio”)
4.
Use the whole numbers in the mole ratio for subscripts in the empirical formula.
B. From masses of elements.
Begin at Step #2 above.
e.g.
What is the empirical formula of a compound that is 34.5 % iron and 65.5 % chlorine?
1. Assume 100 g  34.5 g Fe, 65.5 g Cl
2. Find moles of each.
34.5 g Fe x 1 mol Fe
55.85 g Fe
= 0.618 mole Fe
65.5 g Cl x 1 mol Cl
35.45 g Cl
= 1.84 mole Cl
3. Divide each of the moles by the smaller of the moles. Approximate to the nearest whole numbers.
For Fe: 0.618 mol = 1.00
0.618 mol
For Cl: 1.84 mol = 2.98 ~ 3.00
0.618 mol
 Mole ratio: 1 Fe: 3 Cl
4. Use the lowest whole number ratio as subscripts in the empirical formula.
Fe1Cl3 = FeCl3
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Moles WS 3
Empirical Formulas Practice Problems
Show your work.
1. Find the empirical formula for the compounds that have the following mass percent composition.
a. 20.23 % Al and 79.77% Cl
b. 24.74 % K, 34 % Mn and 40.50 % O
2. Iron and oxygen combine to form two different oxides. Given the following information, find the
empirical formula of each
a. Compound 1: 77.7 % Fe and 22.3 % O
b. Compound 2: 69.9 % Fe and 30.1 % O
3. Find the empirical formula for the following compounds:
a. 3.611 g Ca and 6.389 g Cl
b. 0.461 g C and 0.039 g H
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Moles WS 3
More Empirical Formulas Practice Problems
Reminder: Steps:
1.
2.
3.
4.
Assume a 100 g sample – use same numbers as grams rather than % .
Perform mass  mole conversions.
Divide each result (moles) by the smallest result present (mole ratio).
Look for whole number ratio. e.g. If you obtain a ratio of 0.5:1, you will have to double the values to
get a 1:2 ratio. If you obtain ratios involving thirds, be sure to triple the values (e.g. 1.33:1 X 3  4:3)
Rhyme to remember order of steps to convert % composition  empirical formula:
Percent to mass
Mass to mole
Divide by small
Times till whole
Calculate the empirical formula for a compound that is:
(Box your answers.)
1. 75.0% carbon and 25.0% hydrogen
2. 20.0% magnesium, 26.7% sulfur, and 53.3% oxygen
3. 46.67% lithium and 53.33% oxygen
4. 34.78% iron and 65.22% chlorine
5. 10.83 grams of zinc combine with sulfur to form 16.16 grams of a compound. What is the empirical
formula of this compound? (Hint: First subtract grams of zinc from grams of compound to find mass
of sulfur.)
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