An unknown organic compound is 40.00% C, 6.67% H and 53.33% O by mass. The formula weight of the unknown is 90.0g/mole. Find the empirical and molecular formulas for the unknown compound.

Answer 1

Take a 100 g sample which gives you
40.00 g C
6.67 g H
53.33 O
Convert to mols.
40.00/atomic mass C = ?
6.67/atomic mass H = ?
53.33/atomic mass O = ?

Now find the ratio of thre elements to one another with the smallest number being 1.000. The easy way to do that is to divide the smallest number by itself, then divide the other two numbers by the same small number. That will give you the empirical formula. Calculate the mass of the empirical formula. Then empirical formula mass x ?whole number = 90.0. Solve for the whole number (if it isn't EXACTLy whole then round to the nearest whole number, and multiply the empirical formula by that number; i.e., (CxHyOz)_{whole number}
I believe that whole number will be 3 and that gives you the molecular formula.