Molecular formula

The molecular formula shows the actual number of atoms of each element in one molecule of a substance. It is the empirical formula, or a whole-number multiple of it, for example C₆H₁₂O₆. Malay: formula molekul · Chinese: 分子式.

While the empirical formula gives only the simplest ratio, the molecular formula tells you exactly how many atoms of each element make up one real molecule. To find it you need two things: the empirical formula and the relative molecular mass. You divide the relative molecular mass by the empirical formula mass to find the multiplying factor, then multiply the empirical formula by it.

Example. Ethane has the empirical formula CH₃, whose empirical formula mass is 15. Its relative molecular mass is 30, so the multiplying factor is 30 ÷ 15 = 2, giving the molecular formula C₂H₆. Glucose is a second case: its empirical formula CH₂O has an empirical formula mass of 30, and because its relative molecular mass is 180, the factor is 180 ÷ 30 = 6, giving the molecular formula C₆H₁₂O₆. Water, by contrast, has the empirical formula and molecular formula H₂O, because the simplest ratio already describes the real molecule.

Confusion to avoid. Do not confuse the molecular formula with the empirical formula. They are equal only when the simplest ratio already matches the real molecule, as in water, H₂O. In many compounds, though, the molecular formula is a multiple of the empirical one, so you must use the relative molecular mass to decide the factor rather than guessing.

In this chapter the molecular formula is the natural next step after the empirical formula, and it depends on the relative molecular mass you learned to calculate earlier, so it ties several ideas of SPM Chemistry together into one calculation.