Empirical vs molecular formulas

4 minAS Level2.3.3

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can define empirical formula and molecular formula, convert a molecular formula into an empirical one, and use $M_r$ to go back the other way.

Same recipe, different batch size

Glucose is $\text{C}_6\text{H}_{12}\text{O}_6$ . Ethanoic acid (the acid in vinegar) is $\text{C}_2\text{H}_4\text{O}_2$ . Methanal, used to preserve specimens, is $\text{CH}_2\text{O}$ .

Three very different molecules, and all three have atoms in the same ratio, $1 : 2 : 1$ . That ratio is the empirical formula, $\text{CH}_2\text{O}$ . The actual atom count in each molecule is the molecular formula.

Definition

Empirical formula

The simplest whole-number ratio of the atoms of each element present in a compound.

Definition

Molecular formula

The actual number of atoms of each element present in one molecule of a compound.

Molecular → empirical (easy direction)

Divide every subscript by their highest common factor.

compound	molecular formula	divide by	empirical formula
butane	$\text{C}_4\text{H}_{10}$	2	$\text{C}_2\text{H}_5$
benzene	$\text{C}_6\text{H}_6$	6	CH
hydrogen peroxide	$\text{H}_2\text{O}_2$	2	HO
water	$\text{H}_2\text{O}$	1	$\text{H}_2\text{O}$ (already simplest)

Quick check

DefinitionQuestion 1 · Recall

One-line answerQuestion 2 · Recall

Multiple choiceQuestion 3 · Apply

Answer the 3 questions above to continue.