The formula c2h4 can be classified as
(1) a structural formula, only
(2) a molecular formula, only
(3) both a structural formula and an empirical formula
(4) both a molecular formula and an empirical formula

The answer is 4. Molecular formulas express how many atoms of every element are in a compound, and empirical formulas express the reduced share of elements in a compound. If a compound's molecular formula cannot be made lower any longer, then the empirical formula is the similar as the molecular formula.