Empirical Formula Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Empirical Formula.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The chemical formula that shows the simplest whole-number ratio of atoms of each element present in a compound, obtained by dividing all subscripts by their.

The reduced fraction of atoms—smallest numbers that show the ratio.

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How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Empirical Formula starts with the given amount, names the substance, and chooses the conversion factor that cancels the old unit.

Common stuck point: Students often know a formula related to empirical formula but skip the recognition step: Am I using a mole bridge, molar mass, formula ratio, or balanced-equation ratio to connect measured amounts? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using a mole bridge, molar mass, formula ratio, or balanced-equation ratio to connect measured amounts?

Worked Examples

Example 1

medium
A compound contains 40.0%40.0\% C, 6.7%6.7\% H, and 53.3%53.3\% O by mass. Determine the empirical formula.

Answer

CH2O\text{CH}_2\text{O}

First step

1
Assume 100100 g: C = 40.040.0 g, H = 6.76.7 g, O = 53.353.3 g.

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Example 2

hard
A compound has empirical formula CH2O\text{CH}_2\text{O} and a molar mass of 180 g/mol180\,\text{g/mol}. Find the molecular formula.

Example 3

medium
A compound is 50.0%50.0\% S and 50.0%50.0\% O by mass. Find its empirical formula (S =32= 32, O =16= 16).

Example 4

medium
A compound's molecular formula is C6H12O6\text{C}_6\text{H}_{12}\text{O}_6. Find its empirical formula.

Example 5

medium
A compound is 47.0%47.0\% K, 14.5%14.5\% C, and 38.5%38.5\% O. Find the empirical formula (K =39=39, C =12=12, O =16=16).

Example 6

hard
A compound contains 26.6%26.6\% K, 35.4%35.4\% Cr, and 38.0%38.0\% O. Find the empirical formula (K =39.1=39.1, Cr =52.0=52.0, O =16=16).

Example 7

hard
Combustion of a compound containing only C, H, and O yields mol C=0.10\text{mol}\,\text{C} = 0.10, mol H=0.20\text{mol}\,\text{H} = 0.20, mol O=0.10\text{mol}\,\text{O} = 0.10 (after subtracting CO2_2 and H2_2O contributions). Find the empirical formula.

Example 8

challenge
A hydrocarbon's empirical formula is CH2\text{CH}_2 and its molar mass is 70 g/mol70\,\text{g/mol}. Find the molecular formula and name a possible molecule.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
A compound is 72.0%72.0\% Mg and 28.0%28.0\% N. Find the empirical formula.

Example 2

medium
A compound is 40.0%40.0\% carbon, 6.7%6.7\% hydrogen, and 53.3%53.3\% oxygen by mass. What is its empirical formula?

Example 3

easy
A compound has 2 carbon atoms and 4 hydrogen atoms per molecule. What is its empirical formula?

Example 4

easy
What is the empirical formula of C6H6 (benzene)?

Example 5

easy
What is the empirical formula of H2O2 (hydrogen peroxide)?

Example 6

easy
Glucose is C6H12O6. What is its empirical formula?

Example 7

easy
A compound is found to have a mole ratio of N : O = 1 : 2. What is its empirical formula?

Example 8

easy
A sample contains 0.5 mol Na and 0.5 mol Cl. What is the empirical formula?

Example 9

easy
If subscripts come out as a ratio of 1 : 1.5, what should you do?

Example 10

easy
What is the empirical formula of C4H10?

Example 11

medium
A compound is 40% C, 6.7% H, 53.3% O by mass. Find its empirical formula (use C=12, H=1, O=16).

Example 12

medium
A 100 g sample is 27.3% C and 72.7% O. Find the empirical formula (C=12, O=16).

Example 13

medium
A compound has 1.0 mol C, 1.0 mol H, and a 1:1.5 ratio shows up for O relative to C. Resolve to whole numbers.

Example 14

medium
A 56 g sample of a calcium oxide contains 40 g of calcium (Ca=40) and the rest oxygen (O=16). Find the empirical formula.

Example 15

medium
A compound is 52.2% C, 13.0% H, 34.8% O. Find the empirical formula (C=12, H=1, O=16).

Example 16

medium
A hydrocarbon is 80% C and 20% H by mass. Find its empirical formula (C=12, H=1).

Example 17

medium
If percent composition of C and H sums to only 60%, what likely makes up the remaining 40%?

Example 18

medium
A sample contains 24 g of carbon and 8 g of hydrogen. Find its empirical formula (C=12, H=1).

Example 19

medium
A compound is 75% C and 25% H by mass. Find the empirical formula (C=12, H=1).

Example 20

challenge
Combustion of a hydrocarbon gives 0.6 mol CO2 and 0.6 mol H2O. Find the empirical formula.

Example 21

challenge
A sample contains 11.2 g of iron and 4.8 g of oxygen. Find its empirical formula (Fe=56, O=16).

Example 22

challenge
A compound is 43.7% P and 56.3% O by mass. Find the empirical formula (P=31, O=16).

Example 23

easy
Reduce the molecular formula C4H8\text{C}_4\text{H}_8 to its empirical formula.

Example 24

easy
A compound contains 0.20 mol0.20\,\text{mol} Na and 0.20 mol0.20\,\text{mol} Cl. What is its empirical formula?

Example 25

easy
Reduce C8H18\text{C}_8\text{H}_{18} to its empirical formula.

Example 26

easy
What is the empirical formula of acetylene, C2H2\text{C}_2\text{H}_2?

Example 27

easy
Is the empirical formula of H2O\text{H}_2\text{O} equal to its molecular formula?

Example 28

medium
A compound is 74.2%74.2\% Na and 25.8%25.8\% O by mass. Find the empirical formula (Na =23= 23, O =16= 16).

Example 29

medium
A compound has CH2\text{CH}_2 as its empirical formula and molar mass 42 g/mol42\,\text{g/mol}. Find the molecular formula.

Example 30

medium
A 4.00 g sample contains 1.20 g of C, 0.20 g of H, and 1.60 g of O. Find the empirical formula (C =12=12, H =1=1, O =16=16).

Example 31

medium
A 100 g sample is 30.4%30.4\% N and 69.6%69.6\% O. Find the empirical formula (N =14=14, O =16=16).

Example 32

medium
Combustion of a hydrocarbon produces 0.40 mol CO2\text{CO}_2 and 0.50 mol H2O\text{H}_2\text{O}. Find the empirical formula.

Example 33

hard
A hydrocarbon has empirical formula CH\text{CH} and molar mass 78 g/mol78\,\text{g/mol}. Identify the molecular formula.

Example 34

hard
A compound has 40.0%40.0\% Ca, 12.0%12.0\% C, and 48.0%48.0\% O by mass. Find the empirical formula (Ca =40=40, C =12=12, O =16=16).

Example 35

hard
An oxide of iron has 69.9%69.9\% Fe and 30.1%30.1\% O by mass. Find the empirical formula (Fe =56=56, O =16=16).

Example 36

hard
A sample of an oxide of nitrogen weighs 2.30 g and contains 0.70 g N. Find the empirical formula (N =14=14, O =16=16).

Example 37

challenge
A compound contains 27.4%27.4\% Na, 1.20%1.20\% H, 14.3%14.3\% C, and 57.1%57.1\% O. Find the empirical formula (Na =23=23, H =1=1, C =12=12, O =16=16).
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Background Knowledge

These ideas may be useful before you work through the harder examples.

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