The empirical formula expresses

• A. The simplest whole-number ratio of elements in a compound
• B. The total number of atoms in a molecule
• C. The balanced reaction equation
• D. The mass percent of each element

Answer: (A)

The empirical formula expresses the simplest whole-number ratio of the elements in a compound. It comes from converting the percent composition to moles for each element and then reducing those mole amounts to the smallest whole-number ratio. It doesn’t tell you how many atoms are in a single molecule (that’s the molecular formula), it isn’t a balanced chemical equation, and it doesn’t specify the mass percentages themselves. For example, from a 40% C, 6.67% H, and 53.33% O composition, converting to moles gives roughly 3.33 C, 6.62 H, and 3.33 O; dividing by the smallest (3.33) yields a ratio of about 1 C : 2 H : 1 O, so the empirical formula is CH2O. If you know the compound’s molar mass, you can multiply this ratio to obtain the actual molecular formula.