Key Formulas

These are the key formulas (and other quick methods) that you really need to know:

Name Formula
Moles in solids OR
Moles in gases
Moles in liquids
Concentration of a liquid
Relative atomic mass
  1. Multipy atomic mass by abundance of each isotope.
  2. Add products together.
  3. Divide by 100.
Empirical Formula
  1. Write down elements present as headings.
  2. Write down the mass of each beneath.
  3. Work out the moles of each (divide mass by Ar)
  4. Divide all mol values by the lowest mol value.
  5. Ratio up or down to get whole numbers.
Molecular Formula
  1. Work out mass of empirical formula.
  2. Divide molar mass by new mass.
  3. Times empirical formula by this new number.
Ionic Compounds
  1. Swap the numbers around.
  2. Ignore any 1s.
  3. Ignore equal numbers.
First ionisation energy Mg(g) ----> Mg(g)  + e
Second ionisation energy Mg(g) ----> Mg²(g)  + e
Electron arrangement 1s² 2s² 2p3s² 3p4s² 3d, 4p

Back to the Top