Advanced Placement (AP) Chemistry Practice Exam

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What is an empirical formula?

The actual formula for a substance
The simplest ratio of molecules in a compound
The molecular weight of a compound
A formula representing all isotopes of an element

The correct answer is: The simplest ratio of molecules in a compound

The empirical formula represents the simplest whole-number ratio of elements within a compound. It provides a way to express the composition of a substance in the most reduced form, reflecting the proportion of each element present. For example, the empirical formula for glucose (C₆H₁₂O₆) is CH₂O, which simplifies the ratios of carbon, hydrogen, and oxygen to 1:2:1, indicating the relative number of atoms rather than the exact number of atoms in the molecule. This concept is fundamental in chemistry as it aids in understanding the basic composition of compounds without delving into their complexity. It’s particularly useful in determining the composition of substances when a molecular formula is not available. This distinction is crucial to grasp because knowing the empirical formula allows chemists to derive various calculations related to the behavior and interactions of materials based on their elemental makeup.