What are empirical and molecular formulas?
An empirical formula represents the simplest whole-number ratio of atoms in a compound. It shows the relative number of atoms of each element present, but not the actual number of atoms. For example, the empirical formula of glucose (C₆H₁₂O₆) is CH₂O, which shows the 1:2:1 ratio of carbon, hydrogen, and oxygen atoms.
A molecular formula shows the actual number of atoms of each element in a molecule. It can be the same as the empirical formula or a multiple of it. To find the molecular formula, you need to know the molar mass of the compound.
How to use this calculator
Percentage Mode: Enter the percentage by mass of each element in the compound. The calculator assumes a 100g sample.
Mass Mode: Enter the actual mass of each element in grams.
Molar Mass (Optional): Enter the molar mass to calculate the molecular formula in addition to the empirical formula.
Example calculations
Example 1: A compound contains 48.64% C, 8.16% H, and 43.20% O by mass.
Example 2: A compound contains 2.4g C, 0.4g H, and 3.2g O.
Example 3: A compound with molar mass 78.11 g/mol contains 1.384 g of C and 0.116 g of H.
Steps to calculate empirical and molecular formulas
- Convert to masses: If using percentages, assume 100g sample to convert percentages to grams.
- Convert to moles: Divide each mass by the atomic weight of the element.
- Find ratios: Divide all mole values by the smallest mole value.
- Convert to whole numbers: Multiply all ratios by the smallest integer that makes them whole numbers.
- Write empirical formula: Use the whole number ratios as subscripts in the empirical formula.
- Calculate molecular formula (if molar mass provided): Divide the molar mass by the empirical formula mass to get the multiplier, then multiply the empirical formula by this factor.
