Using Empirical Information to Calculate Molecular Formulas

In a previous article I wrote about how to calculate an empirical formula from mass percent compositions. In this article I will explain how you can use the information from an empirical formula to calculate the actual molecular formula of the compound. These topics are always present on general chemistry exams, and are important to master sufficiently.

To review, an empirical formula is the simplest way of describing a compound. The molecular formula however, is the exact composition of this compound and is some multiple of the empirical formula.

If you know the empirical formula and the molar mass, and you are asked to find the molecular formula you must first calculate the molar mass of the empirical formula. Once you know this, you divide the molar mass of the compound by this to get some number. If you have done your calculations correctly it should be an integer number. To get the molecular formula you multiply each element in the empirical formula by the integer from the division.

To show an example, suppose you are given the empirical formula as CH2O, the molar mass of 60 moles/gram, and the problem asks you to find the molecular formula. You calculate the molar mass of CH2O to be (12+2+16)=30 g/mol. When you divide the known molar mass by 30, you get 2 as an integer number. You multiply each element in CH2O by two to get the molecular formula of C2H4O2 (acetic acid).

There are many variations of this problem, and chances are that you must first find the empirical formula using percent composition. As a check you should always calculate the molar mass of your molecular formula by adding up the individual molar masses of each element to see that it matches with what was given in the problem.

In some cases the molecular formula comes out to be the same as the empirical formula, and this is perfectly good. Consider a water molecule, H2O. This formula is both the empirical and molecular formula because you cannot break down the ratios further without going into fractions. In certain topics in chemistry such as thermochemistry fractional formulas are OK, however in molecular formulas of a compound you should always get integer multiples of the elements.