Final answer:
To determine the molecular formula from the empirical formula, you need the molar mass of the compound. You then divide the molar mass by the empirical formula mass to find the number of empirical units in the molecular formula.
Step-by-step explanation:
The information needed to determine the molecular formula of a compound from the empirical formula is the compound's molar mass.
To derive the molecular formula, the molar mass of the compound is divided by the mass of the empirical formula—that is, the sum of the atomic masses for all atoms in the empirical formula. If, for instance, the molar mass is twice as much as the empirical formula mass, then the molecular formula contains two empirical formula units.
To clarify this process, suppose you have a compound with an empirical formula CH2 and a molar mass of 56 g/mol. The empirical formula mass (12 g/mol for carbon + 2*1 g/mol for hydrogen) is 14 g/mol.
The molecular formula is found by dividing the molar mass by the empirical formula mass (56 g/mol ÷ 14 g/mol = 4), which indicates there are four empirical units in the molecular formula, giving us C4H8 as the molecular formula.