Calculating Molecular Formula

Once you have an empirical formula, you can calculate the molecular formula if you know the molar mass. Here’s a sample problem:

A compound has the empirical formula CH₂O. If the molar mass is approximately 180 g/mol, what is the molecular formula?

To solve this problem we need to remember that the molecular formula must be a multiple of the empirical formula (since empirical is the simplest ratio). That means that the molar mass must be a multiple (the same multiple) of the empirical mass. If that’s enough info for you to figure it out, great.

If not, here is a way to organize yourself: Set up a table like this:

When filled in, the table will hold the following information:

So, let's fill in what we know:

Our first step, then, is to calculate the empirical mass. (We can do this as an approximation)

Now we need to find the ratio between the molecular (or molar) mass and the empirical mass

Lastly, we multiple the empirical formula by the value we just found to give the molecular formula

Let's try one more.