A Level Chemistry OCR Practice Exam

The characterization of an empirical formula refers to the simplest whole-number ratio of the elements in a compound. In this context, CH2 is indeed an empirical formula for ethylene, as it represents the simplest ratio of carbon to hydrogen in that compound. Ethylene’s molecular formula is C2H4, which can be simplified to the empirical formula CH2 by dividing both subscripts by 2.

An empirical formula gives a basic understanding of the composition of a substance, without revealing the actual number of atoms in the molecule. In contrast, glucose (C6H12O6) and propane (C3H8) both have molecular formulas that cannot be expressed in simpler whole-number ratios without losing information about the actual composition, as they are already in their simplest ratio specific to their unique molecular structure. While NaCl is a correct representation of a substance's empirical formula, it is also the molecular formula for sodium chloride, hence it doesn't demonstrate the concept as clearly as option B does.