Three of the following formulas might be either an empirical or a molecular formula. The formula that must be a molecular formula is (a) \(\mathrm{N}_{2} \mathrm{O} ;\) (b) \(\mathrm{N}_{2} \mathrm{H}_{4}\) (c) \(\mathrm{NaCl} ;\) (d) \(\mathrm{NH}_{3}\)

Consider each of the provided formulas: (a) \(\mathrm{N}_{2} \mathrm{O}\), (b) \(\mathrm{N}_{2} \mathrm{H}_{4}\), (c) \(\mathrm{NaCl}\), (d) \(\mathrm{NH}_{3}\). Each of these represents a compound composed of different elements.

Analyze each formula to determine if it could be an empirical formula, i.e., the simplest, reduced ratio of elements, or if it must be a molecular formula, i.e., the actual number of atoms in a molecule of the compound. (a) \(\mathrm{N}_{2} \mathrm{O}\) - This could be an empirical formula because it exhibits a whole number ratio of elements. Also, it could be a molecular formula representing the exact number of atoms in molecular nitrogen monoxide.(b) \(\mathrm{N}_{2} \mathrm{H}_{4}\) - This could also be an empirical formula because it exhibits a whole number ratio of elements. Also, it could be a molecular formula representing the exact number of atoms in a molecule of hydrazine.(c) \(\mathrm{NaCl}\) - Similarly, this could be an empirical or molecular formula because it represents a 1:1 ratio of sodium to chlorine atoms, as in a molecule of sodium chloride.(d) \(\mathrm{NH}_{3}\) - Evidently, this must be a molecular formula representing the exact number of atoms in a molecule of ammonia, since there's no simpler whole number ratio for \(\mathrm{NH}_{3}\).

After analyzing each option, it can be identified that option (d) \(\mathrm{NH}_{3}\) must definitely be a molecular formula as it represents the exact composition of ammonia, with no further scope for simplification.