6.1 Empirical Formula and Molecular Formula

This topic introduces two important types of chemical formulas — the empirical formula and the molecular formula — and explains the difference between them.

Definition

Empirical Formula

The empirical formula of a compound is the chemical formula that gives the simplest integer ratio of the atoms of each element.

Simple explanation: The empirical formula tells you the simplest whole-number ratio of atoms in a compound.

Molecular Formula

A molecular formula is an expression that specifies the number of atoms of each element in one molecule of a compound. This shows the actual number of atoms in each molecule.

Simple explanation: The molecular formula tells you exactly how many atoms of each element are in one molecule.

Main Idea

Remember that the chemical formula of a compound tells you which elements it contains and the whole number ratio of those atoms. In a chemical formula, the elemental symbol and numerical subscripts indicate the type and number of each atom in the compound.

The compound has several chemical formulas. Learn about the two types of chemical formulas: empirical and molecular.

Simple explanation: A compound can be represented by different formulas — one showing the simplest ratio (empirical) and one showing the actual number of atoms (molecular).

Core Concept

Detailed Explanation

Empirical Formula — In Detail

The empirical formula of a compound is the chemical formula that gives the simplest integer ratio of the atoms of each element.

Example 1: Hydrogen Peroxide

The compound hydrogen peroxide has one H atom for every O atom. Therefore, the simplest ratio of hydrogen to oxygen is 1:1. The empirical formula for hydrogen peroxide is therefore written as HO.

Example 2: Glucose

The simplest ratio between C, H, and O atoms in glucose is 1:2:1. What is the empirical formula of glucose? Answer: CH₂O

Simple explanation: The empirical formula is the "reduced" version of the molecular formula — like reducing a fraction to its simplest terms.

Molecular Formula — In Detail

A molecular formula is an expression that specifies the number of atoms of each element in one molecule of a compound. This shows the actual number of atoms in each molecule.

Example 1: Hydrogen Peroxide

• two hydrogen atoms
• two oxygen atoms

Example 2: Glucose

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

Simple explanation: The molecular formula is the "full" formula that tells you exactly how many atoms of each element are in one molecule.

Benzene — Example from the Book

Benzene is a compound of carbon and hydrogen. It contains one C atom for every H atom. Each benzene molecule actually has six carbon atoms and six hydrogen atoms. Identify the empirical and molecular formulas of benzene from the following formulas:

C₆H₆ and CH

• Empirical formula: CH (simplest ratio 1:1)
• Molecular formula: C₆H₆ (actual number of atoms)

Simple explanation: Benzene's empirical formula is CH, but its molecular formula is C₆H₆ — six times the empirical formula.

Water and Carbon Dioxide — Example from the Book

Molecular formulas for water and carbon dioxide are H₂O and CO₂, respectively. What are their empirical formulas?

• Water (H₂O): The ratio of H to O is 2:1, which is already the simplest whole-number ratio → Empirical formula = H₂O
• Carbon dioxide (CO₂): The ratio of C to O is 1:2, which is already the simplest whole-number ratio → Empirical formula = CO₂

Simple explanation: When the molecular formula is already in the simplest ratio, the empirical and molecular formulas are the same.

Intermediate Level

Relationship Between Empirical and Molecular Formulas

The molecular formula is always a whole-number multiple of the empirical formula:

Molecular formula = n × (Empirical formula)

Where n is a whole number (1, 2, 3, ...).

Examples:

Simple explanation: If you multiply the empirical formula by a whole number, you get the molecular formula.

How to Determine the Empirical Formula from the Molecular Formula

• Write the molecular formula.
• Find the greatest common divisor of the subscripts.
• Divide all subscripts by this number.
• Write the empirical formula.

• Subscripts: 6, 12, 6
• GCD = 6
• Divide: 66 = 1, 126 = 2, 66 = 1
• Empirical formula: CH₂O

How to Determine the Molecular Formula from the Empirical Formula

To find the molecular formula, you need the molar mass of the compound:

n = Molar mass of compoundMolar mass of empirical formula

Then: Molecular formula = n × (Empirical formula)

• Molar mass of CH₂O = 12 + 2(1) + 16 = 30 g/mol
• n = 18030 = 6
• Molecular formula = 6 × CH₂O = **C₆H₁₂O₆**

Why Both Formulas Are Important

Important Points

• - Empirical formula: simplest integer ratio of atoms in a compound.
• - Molecular formula: actual number of atoms of each element in one molecule.
• - Examples:
• - Hydrogen peroxide: empirical = HO; molecular = H₂O₂
• - Glucose: empirical = CH₂O; molecular = C₆H₁₂O₆
• - Benzene: empirical = CH; molecular = C₆H₆
• - Water: empirical = H₂O; molecular = H₂O
• - Carbon dioxide: empirical = CO₂; molecular = CO₂
• - Molecular formula = n × empirical formula (where n is a whole number).
• - Sometimes the empirical and molecular formulas are the same (e.g., H₂O, CO₂).

Remember

• - Empirical formula = simplest ratio.
• - Molecular formula = actual number of atoms.
• - Hydrogen peroxide: HO (empirical) → H₂O₂ (molecular).
• - Glucose: CH₂O (empirical) → C₆H₁₂O₆ (molecular).
• - Benzene: CH (empirical) → C₆H₆ (molecular).
• - Water: H₂O (both).
• - Carbon dioxide: CO₂ (both).
• - Molecular formula = n × empirical formula.

Common Mistakes

• 1. Thinking empirical and molecular formulas are always different. For some compounds (H₂O, CO₂), they are the same.
• 2. Thinking the empirical formula gives the actual number of atoms. It gives the simplest ratio.
• 3. Confusing "simplest ratio" with "actual count." Empirical = simplest; molecular = actual.
• 4. Forgetting to reduce to the simplest ratio. Always divide by the greatest common divisor.
• 5. Thinking the molecular formula can be smaller than the empirical formula. The molecular formula is always equal to or a multiple of the empirical formula.
• 6. Thinking all compounds have both formulas. Some compounds (like ionic compounds) have formula units instead of molecules.

Quick Revision

• Empirical formula: the chemical formula that gives the simplest integer ratio of the atoms of each element.
• Molecular formula: an expression that specifies the number of atoms of each element in one molecule of a compound.
• Examples:
• Hydrogen peroxide: empirical = HO; molecular = H₂O₂
• Glucose: empirical = CH₂O; molecular = C₆H₁₂O₆
• Benzene: empirical = CH; molecular = C₆H₆
• Water: empirical = H₂O; molecular = H₂O
• Carbon dioxide: empirical = CO₂; molecular = CO₂
• Relationship: Molecular formula = n × empirical formula (where n = 1, 2, 3, ...).
• Sometimes the empirical and molecular formulas are the same (e.g., H₂O, CO₂).

60-Second Revision

• Empirical formula = simplest ratio.
• Molecular formula = actual number of atoms.
• Hydrogen peroxide: HO → H₂O₂.
• Glucose: CH₂O → C₆H₁₂O₆.
• Water: H₂O → H₂O.
• Warning: empirical and molecular formulas can be the same — but not always.