Question

An unknown substance has the composition of 77.87% c, 11.76% h and 10.37% o. the compound has a molar mass of 154.25 g/mole. what is the molecular formula?

Answer 1

We need to first calculate the empirical formula. Empirical formula is the simplest ratio of whole numbers of components in a compound,
Mass percentages have been given. We need to then calculate for 100 g of the compound 
                               C                     H                           O
mass                    77.87 g              11.76 g                 10.37 g
number of moles  77.87/12            11.76/1                 10.37/16
moles                  = 6.48                  = 11.76                 =0.648
divide by least number of moles 
                              6.48/0.648       11.76/0.648            0.648/0.648
                              = 10                 =18.1                      = 1
rounded off
C - 10 , H - 18 and O - 1
empirical formula - C₁₀H₁₈O
mass of empirical unit = 12 x 10 + 1x 18 + 16 = 120 + 18 + 16 = 154 
number of empirical units = molecular mass / mass of one empirical unit 
                                         = 154.25 / 154 = 1.00 
Therefore molecular formula = C₁₀H₁₈O