Question

A compound is 80.0% carbon and 20.0% hydrogen by mass. assume you have a 100.-g sample of this compound. the molar mass of the compound was found to be 30.069 g/mol. what is the molecular formula?

Answer 1

Basis of the calculation: 100g
 
For Carbon: 
 Mass of carbon = (100 g)(0.80) = 80 g
  Number of moles of carbon = (80 g)(1 mole / 12g) = 20/3

For Hydrogen:
  Mass of hydrogen = (100 g)(0.20) = 20 g
     Number of moles of hydrogen = (20 g)(1 mole / 1 g) = 20

Translating the answer to the formula of the substance,
     C20/3H20

Dividing the answer,
    CH3

The molar mass of the empirical formula is:
    12 + 3 = 15 g/mol

Since, the molar mass given for the molecular formula is 30.069 g/mol, the molecular equation is,
    C2H6

ANSWER: C2H6