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 shows only the simplest ratio.

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 = empirical formula \times\, n (where n is a whole number).

Common stuck point: You need molar mass to find n and get from empirical to molecular.

Sense of Study hint: When finding a molecular formula from experimental data, start with the empirical formula. First convert percent composition to moles of each element. Then find the simplest whole-number ratio (empirical formula). Finally, divide the known molar mass by the empirical formula mass to get n, and multiply all subscripts by n.

Worked Examples

Example 1

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

Solution

  1. 1
    The empirical formula shows the simplest whole-number ratio of atoms (e.g., \text{CH}_2\text{O}).
  2. 2
    The molecular formula shows the actual number of atoms in one molecule (e.g., \text{C}_6\text{H}_{12}\text{O}_6 for glucose).
  3. 3
    The molecular formula is always a whole-number multiple of the empirical formula: \text{C}_6\text{H}_{12}\text{O}_6 = (\text{CH}_2\text{O})_6.

Answer

\text{Empirical: CH}_2\text{O. Molecular: C}_6\text{H}_{12}\text{O}_6\text{ (×6 multiple)}
Some compounds have the same empirical and molecular formula (e.g., water is both \text{H}_2\text{O}). Others differ — acetic acid (\text{C}_2\text{H}_4\text{O}_2) and glucose (\text{C}_6\text{H}_{12}\text{O}_6) share the same empirical formula \text{CH}_2\text{O} but are completely different compounds.

Example 2

medium
A compound has an empirical formula of \text{NO}_2 and a molar mass of 92\,\text{g/mol}. Determine the molecular formula. (N = 14.01, O = 16.00)

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.

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 \text{CH} and molar mass 78\,\text{g/mol}. Find the molecular formula. (C = 12.01, H = 1.008)

Example 2

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

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

empirical formulamolar mass