Final answer:
The molecular formula shows the actual number of atoms in a molecule, while the empirical formula represents the simplest atom ratio. Dividing the molecular formula by the greatest common divisor gives the empirical formula. For specific determination, knowledge of the compound's molar mass is essential.
Step-by-step explanation:
The molecular formula of a compound represents the actual number of atoms of each element in a molecule, while the empirical formula indicates the simplest whole-number ratio of these elements. If a student presents a molecular composition of a compound, the empirical formula can be found by dividing the subscripts in the molecular formula by the greatest common divisor. For instance, if the molecular formula is C₂H₄O₂ (acetic acid), the ratio of atoms is 2:4:2. By dividing by the lowest common denominator (2), we obtain CH₂O as the empirical formula. To determine the molecular formula from an empirical one, one must know the compound's molar mass. For example, the empirical formula for ethene is CH₂, and its molecular formula is C₂H₄, reflecting the actual molecule's composition.
There are instances where the molecular formula is a simple multiple of its empirical counterpart. In the case of benzene, the empirical formula is CH since experimentally it is established that there is one hydrogen atom for each carbon atom in benzene. Additional experiments determining the molecular mass reveal the molecular formula of benzene to be C6H6, with six atoms of carbon and six atoms of hydrogen.