How to determine a molecular formula from an empirical formula?

In order to determine a molecular formula of an unknown compound from its empirical formula, you must know the molecular weight of the compound.

Say you know CH is the empirical formula of some unknown compound. CH could be the empirical formula for benzene (C6H6) or ethyne (C2H2) or several other compounds.

If we discover the molecular formula of our unknown compound to be 78 grams/mol then we can identify our unknown compound as benzene.

Here's how.

1) Calculate the formula weight of our empirical formula.

In this example, CH has a formula weight of 13 grams/mol.

2) Divide the known molecular weight by the empirical formual weight.

We discovered that our unknown compound has a moleculer weight of 78 grams/mole.

78/13 = 6

3) Multiply the subscripts in the emprical formula by this ratio and you will now have a molecular formula.

CH has understood 1's for subscripts.

C1H1 x 6 (remember we are multiplying the subscripts) = C6H6

*The molecular weight of a compound can be found by looking at colligative properties.