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
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.