What distinguishes an empirical formula from a molecular formula?

• A. The empirical formula gives the simplest whole-number ratio of elements.
• B. The molecular formula shows the exact number of each type of atom in a molecule.
• C. The molecular formula is always the same as the empirical formula for compounds with one type of atom.
• D. The empirical formula indicates the molecule's mass.

Answer: (A)

The key idea is that empirical and molecular formulas convey different levels of detail about a compound’s composition. The empirical formula shows the simplest whole-number ratio of the elements present. It reduces the composition to the smallest integers that keep the same proportions. The molecular formula, by contrast, gives the exact number of each type of atom in a single molecule, which is the full count used to determine the molecule’s true molar mass.

This distinction matters because many compounds share the same empirical formula but differ in how many of those units appear in a molecule. For example, a compound with the empirical formula CH2O could be glucose with molecular formula C6H12O6, where the molecular formula is simply a multiple of the empirical formula. The empirical formula doesn’t tell you the mass; you need the molecular formula (or the molar mass and the multiple) to get the actual weight of a molecule.

The other statements aren’t correct because they either describe the molecular formula rather than the difference, or they misstate what the empirical formula indicates. The empirical formula does not indicate molecular mass, and for compounds composed of a single element, the empirical formula is not necessarily the same as the molecular formula.