Determination of molecular formula

The molecular formula of a compound is the formula that states the actual number of atoms of each constituent element present in one molecule of that compound — for example, the molecular formula $C_2H_4Cl_2$ says a single molecule of that compound contains exactly two carbon atoms, four hydrogen atoms and two chlorine atoms. A molecular formula cannot usually be written down directly from a measurement; instead, it is worked out from two experimentally determined quantities: the percent elemental composition of the compound (found by quantitative analysis of each constituent element) and the compound's molar mass (found by an independent, suitable experimental method appropriate to the substance). The percent composition first gives a simpler ratio- …