What Are Chemical Formulae?

A chemical formula shows the types of elements and the number of atoms of each element in a compound.

Chemical formulae are important because they allow chemists to represent substances clearly and accurately using symbols and numbers.

For example:

Water → H₂O
Carbon dioxide → CO₂
Sodium chloride → NaCl

These formulae tell us which elements are present and their ratios in the compound.

Molecular Formula

The molecular formula shows the actual number of atoms of each element present in one molecule of a compound.

Example:

Glucose has the molecular formula:

C₆H₁₂O₆

This means one molecule of glucose contains:

• 6 carbon atoms
• 12 hydrogen atoms
• 6 oxygen atoms

The molecular formula gives the exact composition of the molecule.

Empirical Formula

The empirical formula shows the simplest whole number ratio of atoms of each element in a compound.

Example:

Glucose molecular formula:

C₆H₁₂O₆

Simplifying the ratio:

6 : 12 : 6 → divide by 6 → 1 : 2 : 1

Empirical formula:

CH₂O

So:

Molecular formula → C₆H₁₂O₆
Empirical formula → CH₂O

Difference Between Molecular and Empirical Formula

FeatureMolecular FormulaEmpirical FormulaShowsActual number of atomsSimplest ratio of atomsExampleC₆H₁₂O₆CH₂OPurposeExact compositionSimplified composition

Both types of formulae are important for understanding chemical composition.

Writing Chemical Formulae

Chemical formulae are written using:

• Element symbols
• Subscript numbers

The subscript number indicates the number of atoms of that element in the compound.

Example:

CO₂

This means:

• 1 carbon atom
• 2 oxygen atoms

If no number is written, it means one atom.

Examples of Common Chemical Formulae

CompoundChemical FormulaWaterH₂OCarbon dioxideCO₂MethaneCH₄AmmoniaNH₃Sodium chlorideNaCl

Learning common formulae is essential for solving many chemistry problems.

Importance of Chemical Formulae

Chemical formulae help chemists:

• Understand composition of compounds
• Write chemical equations
• Calculate relative molecular mass
• Perform stoichiometric calculations

They are a fundamental part of chemical communication.

Exam Tip (5070)

Students are often asked to:

• Define empirical formula
• Define molecular formula
• Convert molecular formula to empirical formula

Example exam question:

What is the empirical formula of C₂H₆?

Answer:

2 : 6 → divide by 2 → CH₃

Practice Question

The molecular formula of a compound is C₄H₁₀.

Find its empirical formula.

Answer

4 : 10 → divide by 2 → C₂H₅

Empirical formula = C₂H₅

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