Chemical Formulae


To work out the number of moles in an element/compound, just divide the mass by the Ar or the Mr.


For example:


“How many moles are there in 32g of oxygen?”


Ar of oxygen is 16


So 32/16 = number of moles


Number of moles = 2


To work out the mass, multiply the number of moles by the Ar or Mr.


For example:


“What is the mass of 6 moles of Carbon?”


Ar of Carbon is 12


So 12 x 6 = Mass


Mass= 72g


  • Avagadro's constant = the number of atoms is a mole of an element = 6.02 x 1023
  • To work out the number of particles, just multiply the number of moles by Avagadro's constant

    For example:

    “How many particles are there in 2 moles of carbon?”


    2 x 6.02 x1023

    = 1.2 x 1024


  • Empirical Formula- the simplest ratio of atoms of each element in a compound.
  • Molecular Formula- the actual number of each atom in a molecule.
  • To work out the empirical formulae of a substance, the mass is needed.

    For example:


    “0.020g of Hydrogen combine with 0.168g of Oxygen. What is the empirical formula of Water?”


    Mass:  0.020g H                    0.168g O

    Number of Moles: 0.20/1    0.168/16

    Ratio: 0.02 : 0.01 = 2:1 so the formula is H2O

    To work out the molecular formula, divide the Molecular Mass by the sum of the Atomic masses of the empirical formula.


    For example:

    “The empirical formula of methane is CH3, the formula mass of methane is 30, work out the molecular formula”


    CH3 = 12 + (3x1)

    = 15


    30/15= 2


    so the molecular formula is twice the empirical formula.





    Useful books for revision:

    Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry)
    Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)