The molecular formula of a compound lists all the elements and the number of each element that make up the compound. The simplest formula is similar where elements are all listed, but the numbers correspond to the ratios between the elements. This worked example problem demonstrates how to use the simplest formula of a compound and it's molecular mass to find the molecular formula.
The simplest formula for vitamin C is C3H4O3. Experimental data indicates that the molecular mass of vitamin C is about 180. What is the molecular formula of vitamin C?
First, calculate the sum of the atomic masses for C3H4O3. Look up the atomic masses for the elements from the Periodic Table. The atomic masses are found to be:
H is 1.01
C is 12.01
O is 16.00
Plugging in these numbers, the sum of the atomic masses for C3H4O3 is:
3(12.0) + 4(1.0) + 3(16.0) = 88.0
This means the formula mass of vitamin C is 88.0. Compare the formula mass (88.0) to the approximate molecular mass (180). The molecular mass is twice the formula mass (180/88 = 2.0), so the simplest formula must be multiplied by 2 to get the molecular formula:
molecular formula vitamin C = 2 x C3H4O3 = C6H8O6
An approximate molecular mass is usually sufficient to determine the formula mass, but the calculations tend not to work out 'even' as in this example. You are looking for the closest whole number to multiply by the formula mass to get the molecular mass.