Empirical Formula

The smallest whole number ratio of atoms in a molecule

Eg. Molecular formula of octene = C8H16

Empirical formula of octene = C1H2


Using empirical formula and Mr to find molecular formula

  • calculate Mr of compound from empirical formula

  • multiply until actual Mr is reached to find multiplier

  • multiply no. atoms of each element by number by multiplier


Eg.

NH2 with Mr = 32

(14×1) + (1×2) = 16

16 × 2 = 32

NH2 × 2 = N2H4



Find empirical formula of a compound from mass / percentage

  • divide mass / percentage of each element by its Ar

  • consider smallest value as 1 unit

  • divide other values by unit to calculate ratio


Eg.

3.53 g iron reacts with 6.71 g chlorine

3.53 / 55.8 = 0.063

6.71 / 35.5 = 0.189

0.189 / 0.063 = 3

Fe : Cl = 1 : 3

FeCl3