Percentage composition, empirical and molecular formula

Mass percent of an element = (its mass in one mole of the compound / molar mass of the compound) × 100.

The empirical formula gives the simplest whole-number ratio of atoms; the molecular formula gives the actual number of each kind of atom in a molecule.

Route from percentages to empirical formula: assume 100 g, convert each mass to moles, divide all by the smallest mole value, then multiply by a small integer if needed to reach whole numbers.

Molecular formula = n × (empirical formula), where n = molar mass / empirical formula mass, and n is a whole number.

Glucose (C₆H₁₂O₆) and ethanoic acid (C₂H₄O₂) both have the empirical formula CH₂O, so the empirical formula alone cannot identify a compound.

Background, not stated in this NCERT chapter: if a problem gives vapour density relative to hydrogen, molar mass is about twice that value, a result that follows from Avogadro's law.