Practice Molecular Formula in Chemistry

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick 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.

Showing a random 20 of 50 problems.

Example 1

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 2

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

Example 3

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

Example 4

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 5

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 6

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 7

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 8

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

Example 9

easy
What does a molecular formula tell you about a molecule?

Example 10

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

Example 11

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

Example 12

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)

Example 13

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

Example 14

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

Example 15

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

Example 16

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)

Example 17

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

Example 18

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 19

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 20

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