How would you find the molecular formula from empirical formula?

1 Answer
Jan 31, 2016

Answer:

You need an idea of the molecular mass.

Explanation:

The molecular formula is ALWAYS a multiple of the empirical formula.

Let's say we analyzed napthalene (#C_10H_8#) by combustion. Microanalysis would give an empirical formula of #C_5H_4#; the empirical formula is the simplest whole number of ratio defining constituent atoms in a species (I learnt that definition years ago, and I can still recite it glibly!).

In other experiment (perhaps mass spectroscopy), we could discover that the molecular mass of napthalene was #82# #"amu"#.

So, given that the molecular formula is a multiple of the empirical formula we KNOW that #MF# #=# #(EF)_n#.

From above #EF# #=# #C_5H_4#; and #MF# #=# #(C_5H_4)xxn#, i.e. #n# #=# #2#.