Structure 1.4 – Counting particles by mass : The mole

S1.4.4 – Empirical and molecular formulas

📌 Empirical formula

  • An empirical formula is the simplest ratio of molecules in a compound
  • Avogadro’s number is 6.02 x 1023 and is known as the ‘mole’ (SI unit : mol)
  • One mole of anything consists of 6.02 x 1023 entities (atoms, molecules, ions etc)

Steps to calculate an empirical formula

  1. Divide all the given masses by the atomic masses of their elements
  2. Identify the smallest quotient from the values obtained
  3. Divide all the other quotients by the smallest one to obtain the closest integer values possible
  4. Each integer value correlates to the number of an atom in a compound

Example

QUESTION : A sample of urea contains 1.210g N, 0.161g H, 0.480g C and 0.640g O.
What is the empirical formula of urea?oles of hydrogen atoms are present?

SOLUTION :

  1. N → 1.210/14.01 = 0.086, H → 0.161/1.01 = 0.159, C → 0.480/12.01 = 0.04, O → 0.640/16.00 = 0.04
  2. Smallest quotient : 0.04
  3. 0.086/0.04 ≈ 2, 0.159/0.04 ≈ 4, 0.04/0.04 = 1, 0.04/0.04 = 1
  4. N2H4CO (written as CO(NH2)2)

📌 Molecular formula

  • Molecular formulas are the actual number of atoms in a compound
  • The molecular formula can be found if the empirical formula is known and the molar mass is known
  • First, calculate the mass of the empirical formula and divide the molar mass by this number
  • This will give you an integer that you can then multiply the entire empirical formula by

Example

QUESTION : A compound has the empirical
formula HgCl and a molar mass of 472.08 g mol-‘. What is its molecular formula?

SOLUTION :

Mr HgCl = 236.04 g mol-1

472.08/236.04 = 2

Molecular formula : (HgCl) x 2 = Hg2Cl2