The molecular formula of a compound lists all the elements and the number of atoms of each element that actually 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.
Molecular Formula from Simplest Formula 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?
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
Tips for Working Problems
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.
If you see that the ratio between formula mass and molecular mass is 2.5, you might be looking at a ratio of 2 or 3, but it's more likely you'll need to multiply the formula mass by 5. There's often some trial and error in getting the correct answer. It's a good idea to check your answer by doing the math (sometimes more than one way) to see which value is closest.
If you're using experimental data, there will be some error in your molecular mass calculation. Usually compounds assigned in a lab setting will have ratios of 2 or 3, not high numbers like 5, 6, 8, or 10 (although these values are also possible, especially in a college lab or real world setting).
It's worth pointing out, while chemistry problems are worked using molecular and simplest formulas, real compounds don't always follow the rules. Atoms may share electrons such that ratios of 1.5 (for example) occur. However, use whole number ratios for chemistry homework problems!
Determining the Molecular Formula From Simplest Formula
The simplest formula for butane is C2H5 and its molecular mass is about 60. What is the molecular formula of butane?
First, calculate the sum of the atomic masses for C2H5. 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
Plugging in these numbers, the sum of the atomic masses for C2H5 is:
2(12.0) + 5(1.0) = 29.0
This means the formula mass of butane is 29.0. Compare the formula mass (29.0) to the approximate molecular mass (60). The molecular mass is essentially twice the formula mass (60/29 = 2.1), so the simplest formula must be multiplied by 2 to get the molecular formula:
molecular formula of butane = 2 x C2H5 = C4H10
The molecular formula for butane is C4H10.