The molecular formula for vitamin C is #C_6H_8O_6#. What is the empirical formula?

The thing to remember about a compound's empirical formula is that it represents the smallest whole number ratio that exists between the atoms that form that compound's molecule.

This implies that the molecular formula, which tells you exactly how many atoms of each element you have in a compound's molecule, will always be a multiple of the empirical formula

#color(blue)("molecular formula" = n xx "empirical formula")" "#, where

#n#, an integer, #n>=1#

So, the empirical formula is a sort of building block for the molecular formula. The question now becomes

How many building blocks, i.e. empirical formulas, do you need in order to get the molecular formula?

Well, you know that the molecular formula contains

• six atoms of carbon
• eight atoms of hydrogen
• six atoms of oxygen

What would be the smallest whole number ratio between these numbers?

If you divide all three numbers by #2# you will get

#"For C: " 6/2 = 3#

#"For H: " 8/2 = 4#

#"For O: "6/2 = 3#

Since you cannot longer divide by an integer to get a smaller whole number ratio, the empirical formula of vitamin C will be

#"C"_3"H"_4"O"_3#