Empirical vs Molecular Formula: Step-by-Step from Percent Composition

An empirical formula gives the simplest whole-number atom ratio. A molecular formula gives the actual number of atoms in one molecule. Example: glucose has molecular formula C6H12O6 and empirical formula CH2O. Many exam questions first ask for the empirical formula from composition data, then use molar mass to scale to the molecular formula.

When given percent composition, assume a 100 g sample so each percent becomes grams. Convert each element to moles using atomic mass. This creates the raw mole ratio. Keep enough significant digits during this step to avoid ratio drift later.

Divide all mole values by the smallest one to obtain a relative ratio. If ratios are very close to whole numbers (like 1.99 or 3.01), round appropriately. If ratios are near common fractions (1.5, 1.33, 1.25), multiply all ratios by a small integer (2, 3, or 4) to reach whole numbers.

Use the whole-number ratio as subscripts to write the empirical formula. Calculate empirical formula mass. Then compute n = (molar mass)/(empirical formula mass). If n is close to an integer, multiply every empirical subscript by n to get the molecular formula.

Check whether your final molecular formula reproduces the given percent composition within rounding tolerance. If not, revisit the ratio step; most errors happen there. Also verify that calculated n is chemically reasonable and not a non-integer caused by premature rounding. ChemistryIQ can read your formula-workflow from a photo and help pinpoint where ratio or rounding drift began.