Now let's use percent composition to find the molecular formula. Follow these steps to convert the percent composition to the molecular formula. You should notice that the first 4 steps are what you did to find the empirical formula.

Step 1: Assume you have 100 grams of the substance. This makes the math easier with the percentages.
Step 2: Exchange the % sign with the unit of grams.
Step 3: Convert the grams to moles for each element.
Step 4: Find the smallest whole number ratio of moles for each element.
Step 5: Compare the formula mass of the empirical formula to the mass of the molecular formula by dividing the molecular formula mass by the empirical formula mass to find the factor number.
Step 6: Multiply the subscripts of the empirical formula by the factor number.

Given a molecule that contains 38.7% C, 9.70% H, and 51.6% O and a molecular formula mass of 62.0 grams, we are going to determine the true molecular formula. By now, you can determine the empirical formula. In your notes, determine the empirical formula for this molecule.

What is the empirical formula?

Interactive popup. Assistance may be required.

CH3O

Find the mass of the empirical formula. What is the formula mass of the empirical formula?

Interactive popup. Assistance may be required.

31.0 grams

In Step 5, it states to divide the molecular formula mass by the empirical formula mass. The problem states the molecular formula mass is 62.0 grams, and you determined that the empirical formula mass is 31.0 grams. When we divide these numbers we get:

62.0 grams ÷ 31.0 grams = 2

The factor number is 2. Now multiply each elements subscript by the factor number.

There is only 1 Carbons, so you have 1 × 2 = 2 carbons or C2.
There are 3 Hydrogens, so you have 3 × 2 = 6 hydrogens or H6.
There is only 1 Oxygen, so you have 1 × 2 = 2 oxygens or O2.

The molecular formula is C2H6O2.

### Now You Try It!

Calculate the molecular formula from the percent composition and the molecular formula mass. Work the problems in your notes.

 Problem Answer C = 24.3% H = 4.1% Cl = 71.6 % Molecular mass = 99.0 grams Interactive popup. Assistance may be required. Check Your Answer C2H4Cl2 C = 54.6% H = 9.00% O= 36.4% Molecular mass = 176 grams Interactive popup. Assistance may be required. Check Your Answer C8H16O4