Calculate The Molecular Formula of Each of The Following Compounds

Determining the molecular formula of a compound is essential in chemistry for understanding its composition and properties. This guide explains how to calculate molecular formulas from empirical formulas and percentage compositions, with practical examples and a dedicated calculator.

What is a Molecular Formula?

The molecular formula of a compound shows the actual number of atoms of each element in a molecule. It differs from the empirical formula, which shows the simplest whole number ratio of atoms in a compound.

For example, the molecular formula of glucose is C₆H₁₂O₆, while its empirical formula is CH₂O. The molecular formula provides more detailed information about the compound's structure and properties.

Key difference: Molecular formula shows the actual number of atoms, while empirical formula shows the simplest ratio.

How to Calculate Molecular Formulas

From Empirical Formula and Molar Mass

To find the molecular formula from the empirical formula and molar mass:

Calculate the molar mass of the empirical formula.
Divide the molar mass of the compound by the molar mass of the empirical formula to get the multiplication factor (n).
Multiply each subscript in the empirical formula by n to get the molecular formula.

Formula: Molecular Formula = Empirical Formula × n

Where n = Molar Mass of Compound / Molar Mass of Empirical Formula

From Percentage Composition

When given the percentage composition of a compound:

Assume 100 grams of the compound to simplify calculations.
Convert each percentage to grams.
Divide each gram value by the molar mass of the element to get moles.
Find the simplest whole number ratio of moles to get the empirical formula.
Use the empirical formula and molar mass to find the molecular formula as described above.

Examples of Molecular Formula Calculations

Example 1: From Empirical Formula

Given:

Empirical formula: CH₂O
Molar mass of compound: 180 g/mol

Solution:

Molar mass of empirical formula (CH₂O) = 12 + (2 × 1) + 16 = 30 g/mol
n = 180 / 30 = 6
Molecular formula = CH₂O × 6 = C₆H₁₂O₆

Molecular formula: C₆H₁₂O₆

Example 2: From Percentage Composition

Given:

Compound contains 75% C, 25% H
Molar mass of compound: 78 g/mol

Solution:

Assume 100 g: 75 g C, 25 g H
Moles of C = 75 / 12 = 6.25
Moles of H = 25 / 1 = 25
Simplest ratio: 6.25:25 = 1:4 → Empirical formula CH₄
Molar mass of CH₄ = 16 g/mol
n = 78 / 16 = 4.875 → Round to 5
Molecular formula = CH₄ × 5 = C₅H₂₀

Molecular formula: C₅H₂₀

Frequently Asked Questions

What is the difference between molecular formula and empirical formula?: The molecular formula shows the actual number of atoms in a molecule, while the empirical formula shows the simplest whole number ratio of atoms in a compound.
How do I know when to use the molecular formula vs empirical formula?: Use the molecular formula when you need to know the exact composition of a molecule. Use the empirical formula when you only need the simplest ratio of elements.
Can the molecular formula be determined from mass spectrometry data?: Yes, mass spectrometry can provide information about the molecular weight of a compound, which can help determine the molecular formula when combined with other analytical data.
What if the calculated n value is not a whole number?: If n is not a whole number, you may need to adjust your calculations or consider that the compound might have a more complex structure than initially thought.