Question

What would be the empirical formula of a compound that is 25.5% carbon, 6.40% hydrogen, and 68.1% oxygen?

Answer 1

Empirical formula is the simplest ratio of whole numbers of components in a compound 
in 100 g of compound 
                                C                          H                          O
mass                   25.5 g                     6.40 g                 68.1 g
number of moles  25.5 g/12 g/mol      6.40 g/ 1 g/mol      68.1 g/ 16 g/mol 
                             = 2.13 mol              = 6.40 mol          = 4.26 mol
divide by least number of moles 
                             2.13/2.13 = 1          6.40/2.13 = 3.0    4.26/2.13 = 2.0
all rounded off 
C - 1
H - 3
O - 2

empirical formula - CH₃O₂