How do you determine the molecular formula of a compound from its empirical formula?

If you are given the empirical formula and are asked for the actual formula, then the molecular mass of the compound will be given too. Take this example problem:

Empirical Formula: CH2O

Molecular Mass: 180.0

First you have to find the empirical mass. Just find the atomic masses of all the elements in the empirical formula and add them together. If there are multiple atoms of the same element, then you have to add the element's Atomic Mass for every multiple. In the example, you have to add hydrogen twice because there are two hydrogen atoms in the empirical formula.

C- 12.0

H- 1.00

H- 1.00

O- 16.0

+

_________

30.0

The empirical formula is some multiple of the actual formula. The empirical formula shows the ratio of atoms as 1:2:1. This means that the actual formula could be 2:4:2 or 3:6:3 or 4:8:4 etc. In order to find what multiple it is, divide the molecular mass by the empirical mass. An easier way to think of it is:

x(empirical mass) = molecular mass

Use this formula to find x:

x(30.0) = 180.0

x = 6

In this example, x turned out to be exactly 6, but in some cases (especially in lab results) the answer will be close to a whole number but not exact. There is usually a standard +/- .02 for your result. If the x value was 6.02 or 5.98, we would just round up or down as long as it is within .02

Take the x value and multiply it with the number of atoms in the original empirical formula.

C1H2O1 * 6 = C6H12C6

The final result is the actual molecular formula.