Empirical and molecular formula
The empirical formula is the simplest whole-number ratio of atoms in a compound. The molecular formula is the actual number of atoms. Molecular formula = empirical formula × n, where n = molar mass ÷ empirical mass.
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Analogy
A recipe scaled down to its smallest form: 'two parts flour to one part sugar' hides how big the batch is.
Example
Per 100 g: C 3.33 mol, H 6.6 mol, O 3.33 molDivide by 3.33: C 1, H 2, O 1Empirical formula: CH₂O
In practice
Combustion analysis turns burnt mass of CO₂ and H₂O into an empirical formula for an unknown organic compound.
Practice
Flashcards
- What is the empirical formula?The simplest whole-number ratio of atoms in a compound.
Questions people ask
How do I find the molecular formula from the empirical formula?
Divide the compound's molar mass by the empirical formula mass to get a whole number n. Multiply every subscript by n. If CH₂O (30 g/mol) has a measured molar mass of 180 g/mol, n = 6, so the molecular formula is C₆H₁₂O₆, which is glucose.
What if my ratios are not whole numbers?
Multiply all the ratios by the same small number to make them whole. A ratio of 1 : 1.5 becomes 2 : 3 when doubled. A ratio of 1 : 1.33 becomes 3 : 4 when tripled. Only round if the value is within about 0.1 of a whole number.