If that is what the question said, then the empirical formula of the hydrocarbon is #C_7H_16#. How do we know?
#(i)# We assume a mass of #100*g# of hydrocarbon.
#(ii)# We work out its atomic composition on the basis of the atomic percentages:
#"Moles of carbon"# #-=# #(84.0*g)/(12.01*g*mol^-1)=7*mol#
#"Moles of hydrogen"# #-=# #(16.0*g)/(1.00794*g*mol^-1)=16*mol#
How did I know that there were #16*g# of hydrogen based solely on the GIVEN #"% percentage composition of carbon?"#
On this basis, the empirical formula, the simplest whole number representing constituent atoms in a species is #C_7H_16#. Clearly the molecular formula is identical.
It is a fact that the #"molecular formula"# is always a whole number multiple of the #"empirical formula"#.
Thus #"molecular formula"# #=# #nxx"empirical formula"#. But we have been given an estimate of molecular mass. So.............
So #100*"amu"-=(C_7H_16)xxn#
Thus #100*"amu"-=(7xx12.011+16xx1.00794)*"amu"xxn#
We solve for #n# (how), and find #n-=1#.
#"And thus the molecular formula"-=C_7H_16#.
See here for another example.
