Empirical & Molecular Formula (AQA A-Level Chemistry): Revision Notes

Example: For glucose $C_6H_{12}O_6$, the empirical formula is $CH_2O$, which shows the simplest 1:2:1 ratio of carbon, hydrogen, and oxygen atoms.

Example: For glucose $C_6H_{12}O_6$, the molecular formula shows the actual number of atoms in the molecule. In this case, the molecular formula is 6 times the empirical formula $CH_2O$.

Example: How to calculate the empirical formula from given data

Step 1: Convert masses to moles:

Use the formula:

Where $A_r$ is the relative atomic mass of each element.

Step 2: Divide each mole value by the smallest number of moles:

This gives the simplest ratio.

Step 3: Adjust the ratio to whole numbers:

If the resulting ratio isn't a whole number, multiply all values by a suitable factor to obtain whole numbers (e.g., multiply by 2 if one value is 1.5).

Example: A compound contains 2.4 g of carbon and 0.4 g of hydrogen.

Calculate the empirical formula.

Step 1: Convert masses to moles:

Step 2: Divide each mole value by the smallest number of moles:

Step 3: Adjust the ratio to whole numbers:

The ratio is 1:2, so the empirical formula is CH₂.

Example: The empirical formula of a compound is $CH_2O$ and its molar mass is 180 g/mol

Determine the molecular formula.

Step 1: Find the molar mass of the empirical formula.

Molar mass of

Step 2: Divide the molar mass of the molecular formula by the empirical formula mass to find $n$.

Find $n$:

Step 3: Multiply the empirical formula by $n$ to obtain the molecular formula.

Multiply the empirical formula by 6: