Find Empirical Formula from Percent Composition
Q: A compound is found to have a molar mass of 144.14 g/mol. Upon elemental analysis, it is found to be made up of 50.0% carbon, 5.6% hydrogen and 44.4% oxygen. What is the molecular formula for the compound?
A: Assume that you have 100 g of the substance. Then, you have 50.0 g of carbon, 5.6 g of hydrogen and 44.4 g of oxygen.
To figure out the number of atoms of each that you have, convert these masses into moles:
So, the ratio of C:H:O is 4.16:5.55:2.78. We need an equivalent ratio in integers. One of my favorite techniques to simplifying the ratio is to divide all of the numbers by the smallest number that appears.
To get rid of a “.5” that appears, we can simply double all of the values.
And so we finally have an integer ratio that matches the mole ratio we found earlier. This means we have 3 carbon atoms for every 4 hydrogen atoms and 2 oxygen atoms. In other words, the empirical formula of out compound is C3H4O2. What is this molecule's mass?
We need the molecular mass of the compound to be DOUBLE this, so double the subscripts in the chemical formula. You will find that the molecule C6H8O4 has a molecular mass of 144.14 g/mol as we need.