A compound has an empirical formula of C2H6. What additional piece of evidence is needed to determine the molecular formula?
a. percent composition of carbon
b. mass of the sample
c. relative numbers of atoms of each element
d. molar mass of the compound

Question

A compound has an empirical formula of C2H6. What additional piece of evidence is needed to determine the molecular formula? 
   a. percent composition of carbon 
  b. mass of the sample 
   c. relative numbers of atoms of each element 
   d. molar mass of the compound

anonymous · Answer

I would say the last choice, molar mass of the compound.

Knowing the empirical formula, the "percent composition" and "relative number of atoms of each element" do not give you any more information.  The mass of the sample is quite arbitrary, leaving you with one logical answer.