Specification Point 1.27:
Know that the Mole (mol) is the unit for the amount of a substance

What is the Mole?

Mole:  The mass of substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of 12C. Mole is the unit representing the amount of atoms, ions, or molecules.

  • One Mole is the amount of a substance that contains 6 x 1023  particles (Atoms, Molecules or Formulae) of the substance (6 x 1023  is known as the Avogadro Number).

Example:

1 mole of Sodium (Na) contains  6 x 1023   Atoms of Sodium

1 mole of Hydrogen (H2) contains  6 x 1023  Molecules of Hydrogen

1 mole of Sodium Chloride (NaCl) contains  6 x 1023  Formulae of Sodium Chloride

Specification Point 1.28:
Understand how to carry out calculations involving amount of substance, Relative Atomic Mass (Ar) and Relative Formula Mass (Mr)

Mole Calculations

CALCULATING MOLES

Equation:

Amount in Moles     =     Mass of Substance in grams    ÷   Mr (or Ar)

Example:

Substance Mass Mr Amount
NaOH 80 g 40 ( 80 ÷ 40 ) = 2 moles
CaCO3 25 g 100 ( 25 ÷ 100 ) = 0.25 moles
H2SO4 4.9 g 98 4.9 ÷ 98 ) = 0.05 moles
H2O 108 g 18 ( 108 ÷ 18 ) = 6 moles
CuSO4.5H2O 75 g 250 ( 75 ÷ 250 ) = 0.3 moles

CALCULATING MASS

Equation:

Mass of Substance (grams)     =     Moles    x    Mr

Example:

Substance Amount Mr Mass
H2O 0.5 moles 18 ( 0.5 x 18 ) = 9 g
NaCl 3 moles 58.5 ( 3 x 58.5 ) = 175.5 g
K2CO3 0.2 moles 138 ( 0.2 x 138 ) = 27.6 g
(NH4)2SO4 2.5 moles 132 ( 2.5 x 132 ) = 330 g
MgSO4.7H2O 0.25 moles 246 ( 0.25 x 246 ) = 61.5 g

CALCULATING RELATIVE FORMULA MASS

Equation:

Mr    =     Mass of Substance in Grams     ÷     Moles

Example:

10 moles of Carbon Dioxide has a Mass of 440 g. What is the Relative Formula Mass of Carbon Dioxide?

Relative Formula Mass = Mass ÷ Number of Moles

Relative Formula Mass = 440 ÷ 10 = 44

Relative Formula Mass of Carbon Dioxide = 44

Need help?

Aiming for a Level 9?

See if you’ve got what it takes. Test yourself with our topic questions.

Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.