The calculation of the weight average molecular weight requires that you know the * Weight Fraction* , W i , of each type of molecule (or the fraction of the total weight represented by each type of molecule). You don't know the weight fractions from the original data, but you can calculate them. So, what is the weight fraction? Like any fraction, it's the amount of the thing of interest divided by the total amount. For weight fraction of one type of molecule, it is the weight of that type of molecule divided by the total weight of the sample. Or, expressed in mathematical form, it is N i M i / N i M i . Notice that you have already calculated both N i M i and N i M i when you calculated the number average molecular weight, so you just need to do the division. The weight average molecular is then calculated by taking the sum W i M i . When you add this information to the data table, it looks like this: