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

1 Answer
Jun 1, 2014

The empirical formula is the simplest whole-number ratio of the elements in a compound.

To find the molecular weight of the empirical formula you add up the atomic masses of each element from the periodic table.

Let's say the empirical formula is #C_2H_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.

The empirical mass can be used to find the molecular formula when given the molecular mass. Divide the molecular mass by the empirical mass and multiply the subscripts as necessary.

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_6H_9#.