Acertain hydrocarbon (a compound consisting of carbon and hydrogen) is 82.66% carbon by mass and has a molar mass of 58.12 g. determine the molecular formula of this hydrocarbon.

Firstly need to determine the empirical formula of the hydrocarbon. Empirical formula is the simplest whole number ratio of components of the compound. Molecular formula  is the actual composition of the components in the compound.

percentage of C - 82.66%
percentage of H - (100-82.66) = 17.34 %
in 100 g of compound ;
mass of C - 82.66 g
mass of H - 17.34 g
                                 C                               H
mass in 100 g        82.66 g                      17.34 g
molar mass           12 g/mol                      1 g/mol
number of moles    6.88 mol                     17.34 mol
(mass/molar mass)
divide the number of moles by least number of moles (6.88 mol)
                               6.88 mol/6.88             17.34/6.88
                                1                                 2.52
multiply these by 2 to get a whole number 
C - 1x 2 = 2
H - 2.52 x 2 = 5.04
round off to nearest whole number 
C - 2
H - 5
ratio of C to H is 2:5
empirical formula - C₂H₅
empirical formula mass = 12 g/mol x 2 + 5 * 1 g/mol = 29 g

next have to find how many empirical units are there in the molecular unit 
molecular unit mass = 58.12 g
empirical unit = 29 g
then number of empirical units = 58.12 / 29 = 2
rounded off , number of empirical units = 2
(C₂H₅) * 2 units
molecular formula = C₄H₁₀