Molecular Formula from Simplest Formula

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Problem

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?

Solution

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

Answer

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.

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