Amount of Substance

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Relative atomic and molecular mass

Relative atomic mass - average mass of one atom of an element/one twelfth mass of an atomic of carbon-12

Relative molecular mass - average mass of one molecule/one twelfth mass of an atomic of carbon-12

Avogadro constant

  • the number of atoms in 12g of carbon-12
  • 6.022  x10^23

Mole - the amount of substance that contains 6.022 x10^23 particles

number of moles = mass in grams/relative molecular mass

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Ideal gas equation

Boyle's Law - PV = constant

Charles' Law - P/T = constant

Gay-Lussac's Law - PV/T = constant for fixed mass of gas

Ideal gas equation - PV = nRT

P = pressure - Pa

V = volume - m^3

n = number of moles

R = constant - 8.31 J/K/mol

T = temperature - K

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Empirical and molecular formulae

An empirical formula is one which represents the simplest ratio of the atoms of each element present in a compound


  • find masses of each of the elements present
  • work out number of moles of atoms of each element
  • convert number of moles of each element into a whole number ratio by dividing each number by the smallest number


  • divide the relative molecular mass by the relative mass of the empirical formula
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Moles in solutions


  • tells us how much solute is present in a known volume of solution
  • measured in mol/dm^3
  • number of moles/volume in dm^3


  • (concentration*volume)/1000 when concentration is given in mol/dm^3 and volume in cm^3
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Ionic equations

In some reactions, the equation can be simplified by considering the ions present.


HCl + NaOH ---> NaCl + H2O

Ions present:

  • H+
  • Cl-
  • Na+
  • OH-
  • Na+
  • Cl-

H+ + Cl- + Na+ + OH- ---> Na+ + Cl- + H20

H+ + OH- ---> H2O

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Atom economies and percentage yields

Atom economy = (100*mass of desired product)/total mass of reactants

Percentage yield = (100*number of moles of specified product)/theoretical maximum number of moles of product

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