Find the empirical formula

Question

5. A chlorinated hydrocarbon was found to contain 24.24 % C, 4.04 % H, 71.72 % Cl. The molecular mass was found to be 99.00 g/mole. A) Calculate the empirical formula of the compound

Answer 1

The standard method for determining the empirical formula given the concentration by mass is the following:

1) consider a sample of 100 g.   Then work out the mass of each element.  For this problem, the mass of C would be 24.24 g,  for H,  4.04 g, for Cl,  71.72 g

2) Divide each mass by the appropriate atomic mass to get the number of moles.  For this problem,  for C, the number of moles is 24.24 /12 = 2.02,  for H the number of moles is 4.04 /1 = 4.04 moles,  for Cl, 71.72 / 35.45 = 2.02 moles

3) divide each of these by the smallest one to get the mole ratios.  For this case 2.02/2.02 ; 4.04/2.02 : 2.02/2.02

  This is 1 : 2 : 1.   So the empirical formula is  C H₂ Cl

 

The molar mass of C H₂ Cl is 49.45  .  Dividing this into 99 gives 2.00,  so

 

 The  molecular formula is the double of this  C₂ H₄ Cl₂   (dichlorethene).