empirical formula from percent compisition + molecular?

Question

The first step in identifying an unknown sample, we need to determine the empirical formula from percent composition. For an organic compound containing only C, H and O, the laboratory sent these results:
C 40.00 % H 6.71 % O 53.29 %


The next step is to determine the molecular formula from mass spectrum. The laboratory sent these results:
formula weight was found between 118.10 and 123.10

Answer 1

C = 40% divide by 12 = 3.33333....
H = 6.71 divide by 1 = 6.71
O = 53.29 divide by 16 = 3.330625
The denominators 12 , 1 & 16 are the Mr's of C,H & O respectively.
So the ratios :-
C : H : O :: 3.333333 : 6.71 : 3.330625
divide through by the samllest ratio
C: H : O :: 1 : 2 : 1
So empirical formula is CH2O
The empirical mass is 12 + 2 + 16 = 30
Compare the empirical mass to formula mass
30 x 4 = 120
So the molecular formula is 4 times the empirical formula.

C4H8O4 the molecular formula.