# How can I calculate the molecular weight of the empirical formula?

Let's say the empirical formula is ${C}_{2} {H}_{3}$. To find the carbon you multiply 12.01 x 2 and add it to the mass of the hydrogen, 1.01 x3. The total molecular weight is 27.05 amu. Often this is called the empirical mass.
Using the empirical formula above and a molecular mass of 81.15 amu, find the molecular formula. You would divide 81.15 by 27.05 to get 3. Multiply the subscripts by 3 to get the molecular formula of ${C}_{6} {H}_{9}$.