molecular formula
Summary
A molecular formula gives the actual number of atoms of each element in one molecule, such as C₆H₁₂O₆ for glucose. It is a whole-number multiple of the empirical formula, so you need the molar mass to get from the empirical formula to the molecular formula.
Detail
Meaning: A molecular formula states the actual number of atoms of each element in one molecule, such as C₆H₁₂O₆ for glucose. The empirical formula is the simplest whole-number ratio of those atoms (CH₂O). The molecular formula is the empirical formula multiplied by a whole number. It does not show bonding, so isomers share one molecular formula.
Equation: molecular formula = (empirical formula)n, where n = molar mass ÷ empirical formula mass. n is a unitless whole number. The two masses use the same unit, usually g/mol.
On the MCAT: Questions give percent composition or combustion data and ask for a formula. Assume 100 g, convert masses to moles, divide by the smallest value, and scale ratios like 1.5 to whole numbers (×2). A molar mass, often from mass spectrometry, then gives n. In organic chemistry, the molecular formula also gives the degree of unsaturation.
Common confusion: Students report the empirical formula as the molecular formula. The two match when n = 1 and differ otherwise. Percent composition alone cannot separate C₂H₄ from C₄H₈, and the molar mass settles it. Ionic compounds such as NaCl are written as formula units, not molecules.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.