Molecular Formula Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Molecular 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 showing the actual number of atoms of each element in one molecule of a compound, as opposed to the empirical formula which.

The real count of atoms, not just the ratio — it tells you exactly what the molecule contains.

Read the full concept explanation →

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: Molecular 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 molecular 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

easy
Explain the difference between an empirical formula and a molecular formula. Give an example where they differ.

Answer

Empirical: CH2O. Molecular: C6H12O6 (×6 multiple)\text{Empirical: CH}_2\text{O. Molecular: C}_6\text{H}_{12}\text{O}_6\text{ (×6 multiple)}

First step

1
The empirical formula shows the simplest whole-number ratio of atoms (e.g., CH2O\text{CH}_2\text{O}).

Full solution

  1. 2
    The molecular formula shows the actual number of atoms in one molecule (e.g., C6H12O6\text{C}_6\text{H}_{12}\text{O}_6 for glucose).
  2. 3
    The molecular formula is always a whole-number multiple of the empirical formula: C6H12O6=(CH2O)6\text{C}_6\text{H}_{12}\text{O}_6 = (\text{CH}_2\text{O})_6.
Some compounds have the same empirical and molecular formula (e.g., water is both H2O\text{H}_2\text{O}). Others differ — acetic acid (C2H4O2\text{C}_2\text{H}_4\text{O}_2) and glucose (C6H12O6\text{C}_6\text{H}_{12}\text{O}_6) share the same empirical formula CH2O\text{CH}_2\text{O} but are completely different compounds.

Example 2

medium
A compound has an empirical formula of NO2\text{NO}_2 and a molar mass of 92 g/mol92\,\text{g/mol}. Determine the molecular formula. (N = 14.0114.01, O = 16.0016.00)

Example 3

medium
A compound is 40% C, 6.7% H, 53.3% O by mass. Its molar mass is 180 g/mol. Find the molecular formula.

Example 4

medium
A hydrocarbon contains 82.7%82.7\% C and 17.3%17.3\% H by mass, with molar mass 58 g/mol58\,\text{g/mol}. Find the molecular formula.

Example 5

hard
Combustion of 1.50 g1.50\,\text{g} of a hydrocarbon yields 4.62 g4.62\,\text{g} CO2\text{CO}_2 and 2.52 g2.52\,\text{g} H2O\text{H}_2\text{O}. Molar mass is 30 g/mol30\,\text{g/mol}. Find the molecular formula.

Example 6

hard
A compound is 32.0%32.0\% C, 6.67%6.67\% H, 18.7%18.7\% N, 42.6%42.6\% O with molar mass 75 g/mol75\,\text{g/mol}. Find the molecular formula. (C=12, H=1, N=14, O=16)

Practice Problems

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

Example 1

medium
A compound has empirical formula CH\text{CH} and molar mass 78 g/mol78\,\text{g/mol}. Find the molecular formula. (C = 12.0112.01, H = 1.0081.008)

Example 2

hard
A compound is 85.7%85.7\% C and 14.3%14.3\% H by mass, with a molar mass of 56.0 g/mol56.0\,\text{g/mol}. Determine both the empirical and molecular formulas.

Example 3

easy
The empirical formula is CH2O and the molecular formula is (CH2O) times 6. What is the molecular formula?

Example 4

easy
Empirical formula CH has empirical mass 13. The molar mass is 78. Find n.

Example 5

easy
Empirical formula NO2 (mass 46), molar mass 92. Find the molecular formula.

Example 6

easy
If the molecular formula equals the empirical formula, what is n?

Example 7

easy
Empirical formula CH2, empirical mass 14, molar mass 42. Find the molecular formula.

Example 8

easy
Empirical formula HO (mass 17), molar mass 34. Find the molecular formula.

Example 9

easy
Is NaCl a molecular formula or a formula unit?

Example 10

easy
Empirical formula CH2O (mass 30), molar mass 180. Find n.

Example 11

medium
A compound has empirical formula CH2 and molar mass 70 g/mol. Find the molecular formula (C=12, H=1).

Example 12

medium
A compound has empirical formula NH2 and molar mass 32 g/mol. Find the molecular formula (N=14, H=1).

Example 13

medium
Empirical formula CHO (C=12,H=1,O=16), molar mass 116 g/mol. Find the molecular formula.

Example 14

medium
A compound is 92.3% C, 7.7% H, with molar mass 78 g/mol. Find the molecular formula (C=12, H=1).

Example 15

medium
Empirical formula C2H5 (mass 29), molar mass 58 g/mol. Find the molecular formula.

Example 16

medium
A gas has empirical formula CF2 (C=12, F=19), molar mass 100 g/mol. Find the molecular formula.

Example 17

medium
A compound has empirical formula CH2O and molar mass 60 g/mol. Find the molecular formula (C=12,H=1,O=16).

Example 18

medium
Empirical formula CH3 (mass 15), molar mass 30 g/mol. Find the molecular formula.

Example 19

medium
Empirical formula CHO2 (C=12,H=1,O=16, mass 45), molar mass 90 g/mol. Find the molecular formula.

Example 20

challenge
A compound is 40.0% C, 6.7% H, 53.3% O with molar mass 180 g/mol. Find the molecular formula (C=12,H=1,O=16).

Example 21

challenge
A compound is 30.4% N and 69.6% O with molar mass 92 g/mol. Find the molecular formula (N=14, O=16).

Example 22

challenge
A hydrocarbon is 85.7% C, 14.3% H with molar mass 56 g/mol. Find the molecular formula (C=12, H=1).

Example 23

easy
Empirical formula CH3\text{CH}_3 (mass 1515) has molar mass 30 g/mol30\,\text{g/mol}. Find nn and the molecular formula.

Example 24

easy
Empirical formula CH2\text{CH}_2 has empirical mass 14 g/mol14\,\text{g/mol}. If n=4n = 4, give the molecular formula.

Example 25

easy
Empirical formula P2O5\text{P}_2\text{O}_5 (mass 142142) has molar mass 284284. Find the molecular formula.

Example 26

easy
Empirical formula HO has empirical mass 1717. Molar mass is 34 g/mol34\,\text{g/mol}. Find the molecular formula.

Example 27

easy
Empirical formula NO2\text{NO}_2 (mass 4646), molar mass 138 g/mol138\,\text{g/mol}. Find the molecular formula.

Example 28

medium
A compound is 40.0%40.0\% S and 60.0%60.0\% O by mass with molar mass 80 g/mol80\,\text{g/mol}. Find the molecular formula. (S=32, O=16)

Example 29

medium
A compound is 43.7%43.7\% P and 56.3%56.3\% O with molar mass 284 g/mol284\,\text{g/mol}. Find the molecular formula. (P=31, O=16)

Example 30

medium
Empirical formula CHO2\text{CHO}_2 (mass 4545), molar mass 90 g/mol90\,\text{g/mol}. Find the molecular formula.

Example 31

medium
Empirical formula NH2\text{NH}_2 (mass 1616), molar mass 32 g/mol32\,\text{g/mol}. Find the molecular formula.

Example 32

medium
A compound has molecular formula C6H12O6\text{C}_6\text{H}_{12}\text{O}_6. What is its empirical formula?

Example 33

medium
Is KCl\text{KCl} a molecular formula?

Example 34

medium
A compound contains 52.2%52.2\% C, 13.0%13.0\% H, 34.8%34.8\% O. Its molar mass is 46 g/mol46\,\text{g/mol}. Find the molecular formula. (C=12, H=1, O=16)

Example 35

medium
Empirical formula CH2\text{CH}_2 (mass 1414), molar mass 84 g/mol84\,\text{g/mol}. Find the molecular formula.

Example 36

hard
A compound is 24.3%24.3\% C, 4.1%4.1\% H, 71.6%71.6\% Cl with molar mass 99 g/mol99\,\text{g/mol}. Find the molecular formula. (C=12, H=1, Cl=35.5)

Example 37

hard
A compound contains 54.5%54.5\% C, 9.10%9.10\% H, 36.4%36.4\% O with molar mass 88 g/mol88\,\text{g/mol}. Find the molecular formula. (C=12, H=1, O=16)

Example 38

hard
Vitamin C is 40.9%40.9\% C, 4.58%4.58\% H, 54.5%54.5\% O with molar mass 176 g/mol176\,\text{g/mol}. Find the molecular formula. (C=12, H=1, O=16)

Example 39

hard
A compound has empirical formula C3H4O3\text{C}_3\text{H}_4\text{O}_3 and molar mass 264 g/mol264\,\text{g/mol}. Find the molecular formula.

Example 40

challenge
A gaseous compound has density 1.964 g/L1.964\,\text{g/L} at STP and contains 80.0%80.0\% C and 20.0%20.0\% H. Find the molecular formula. (At STP, molar volume =22.4 L/mol= 22.4\,\text{L/mol}, C=12, H=1)

Example 41

challenge
Caffeine is 49.5%49.5\% C, 5.15%5.15\% H, 28.9%28.9\% N, 16.5%16.5\% O with molar mass 194 g/mol194\,\text{g/mol}. Find the molecular formula. (C=12, H=1, N=14, O=16)
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Background Knowledge

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

empirical formulamolar mass