How to Calculate Molecular Formulas for Compounds

For many chemistry problems, you must have the ability to calculate molecular formulas for compounds (substances having two or more different elements joined together in a fixed ratio). The molecular formula is the exact number of atoms of the different elements in a compound. If you know the terminology and correct solution procedures, calculating molecular formulas can be relatively simple.

Things You'll Need

• Calculator

Instructions

1. Know the terminology used in chemistry to calculate molecular formulas for compounds. You need to know the meaning of the terms compound, mole, atomic weight, molecular weight, empirical formula, percent composition and the use of the Periodic Table of Elements (see Resources). For molecular formula calculation, there are at least two factors needed for the solution of any problem. These can be a combination of the molecular weight of the compound, the empirical formula and the percentange of composition.

2. Learn to calculate the molecular formula when given the empirical formula (the simplest formula of a substance written with the smallest subscripts) and the molecular weight (the sum of the atomic weights of the atoms in a molecule) of the compound. As an example, calculate the molecular formula of a compound that has a molecular weight of 78g/mol and the empirical formula CH.

3. Determine the weight of the empirical formula by using the Periodic Table to look up the atomic weights (average atomic mass for an element) for each element in the compound. You find that carbon (C) = 12.011g/mol and hydrogen (H) = 1.008 g/mol. Add these to get the weight of the empirical formula of the compound as (12.011+1.008) =13.019 g/mol and then round it to the nearest whole number, which is 13 g/mol.

4. Divide the molecular weight of the compound (78 g/mol) by the molecular weight of the empirical formula (13 g/mol.) to get 78/13 =6. This means that the molecular formula of the compound is six times the empirical formula (CH) or C6H6. This is the formula for benzene. You can also write it as (CH)6.

5. Use the percent composition (mass percentages of each element in the compound) and the molecular weight of a compound formula to determine the molecular formula. As an example, find the molecular formula for a compound that is 84.25 percent carbon and 15.75 percent hydrogen and has a molecular weight of 114g/mol.

6. To get the answer to the problem in Step 5, multiply the molecular weight by the percent composition (expressed as decimals). From Step 5, you get that C = 114g/mol x.8425=96.045 g/mol and H= 114g/mol x .1575= 17.955 g/mol

7. Divide each answer in Step 6 by the atomic weight of each element and round to the nearest whole number. The atomic weight for carbon and hydrogen from Step 3 are carbon (C) = 12.011g/mol and hydrogen (H) = 1.008 g/mol. Divide and you get that C= 96.045/12.011= 7.996 rounds to 8 and H= 17.955/1.008= 17.813 rounds to 18. The molecular formula for this compound is C8H18, which is octane.

Resources

• Get information here on periodic table.
• Check here for information on chemistry glossary.