Empirical formula to molecular formula

Molecular formula is the formula that represents the actual numbers of the different elements in the compound.

Example: Hexane, molecular fomula is C₆H₁₄

Hexane is made up of 6 Carbon atoms (black balls) and 14 Hydrogen atoms (white balls).

The empirical formula for hexane is C₃H₇. That is because this is the simplest whole number ratio of carbon to hydrogen.

The mass of the empirical formula of hexane ( C₃H₇) = 43.089 g/mole

The mass of the molecular formula (i.e. molar mass of hexane) = 86.178 g/mole

Calculate the common factor between the mass of the empirical and molecular formulae

The common factor defines the ratio of the masses of the molecular formula to empirical formula. It allows you to determine the molecular formular if the mass of the molecular formula is KNOWN.

Mass of molecular formula / Mass of empirical formula = n

Example: Hexane

Suppose you know the mass of the molecular formula of hexane is 86.178 g/mole and you know the empirical formula is C₃H₇, determine the molecular formula.

First, find the common factor, n.

 86.178 / Mass of empirical formula = n

   Mass of empirical formula = 43.089 g/mole

 86.178 / 43.089 = 2 = n
 

This common factor tells you that in order to obtain the molecular formula from the empirical formula, you need to multiply the number of the different elements in the formula by n.

In the case of hexane, n = 2.

Empirical Formula:  C3H7
 
  multiply by n = 2  becomes

Molecular Formula:  C6H14

Content suitability

BCIT courses: CHEM 0011