Amount of Substance

Amount of Substance

The Mole


Definition of a mole: The amount of substance that contains as many particles as there are in 12g of carbon-12 atoms.


Can also be described using Avogadro’s constant – 6.023 X 1023   (this does not need to be remembered)

MR = Molar Mass.

Molar mass (MR) is the same as relative molecular mass – eg MR of CO2 = 12 + (2 x 16) = 44.

Mass of Susbstance = MR x No. of Moles

No. of Moles = Mass of Substance/MR


Number of Moles = Concentration x Volume (in dm3)

(1dm= 1000cm3)



Ideal Gas Equation


pV = nRT


p = pressure (Pa)

V = volume (m3)

n = number of moles

R = gas constant (8.31 J K-1 mol-1) – this is always given in questions

T = temperature (K)


Conversion Units

1 kPa = 1000 Pa

1 cm3 = 1 x 10-6 m3

1 dm3 = 1×10-3 m3

oC + 273 = K


Remember: When dealing with questions about gases, volume is in m3 – when dealing with questions on concentration of solutions, volume is always in dm3. Try and remember that gases take up a larger volume, so use a larger unit.




Formulas, Yield, Atom Economy

Empirical Formula = Smallest whole number ratio of atoms of each element in a compound

Molecular Formula = Actual number of atoms of each element in a compound

Eg: Hexane – Molecular Formula = C6H14

Empirical Formula = C3H7 (divided by 2)

                           Percentage Yield = Actual Yield            x 100

                                        Theoretical Yield

Atom Economy = Mass of Desired products        x 100

            Total Mass of Reactants