Advanced Placement (AP) Chemistry Practice Exam

How does a molecular formula differ from an empirical formula?

1. The molecular formula is always larger

2. The molecular formula represents the actual number of atoms in a compound

3. The empirical formula has more detail about the structure

4. The molecular formula describes isotopes

The correct answer is: The molecular formula represents the actual number of atoms in a compound

A molecular formula provides the actual number of each type of atom present in a molecule of a compound, reflecting its true composition. For instance, the molecular formula of glucose is C6H12O6, which indicates that a single molecule contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. On the other hand, an empirical formula simplifies this concept by expressing the simplest whole-number ratio of the different types of atoms in the compound. For glucose, the empirical formula is CH2O, indicating the ratio of carbon to hydrogen to oxygen atoms, rather than the exact counts. The other options do not accurately describe the distinctions. While it may be true that the molecular formula is larger in terms of the number of atoms represented, this is not a universal rule, as in some cases they can be the same (like in compounds that cannot be further reduced, such as H2O, where both formulas are the same). The empirical formula does not provide detailed structural information about the arrangement of atoms, which is not covered by either formula. Lastly, molecular formulas do not address isotopes, which pertain to variations of atoms based on their neutron count, rather than their molecular structure or ratios.