Summary IGCSE Chemistry- Topic 4: Stoichiometry

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Amount of Substance

Avogadro’s Constant : is the number of particle in 1 mole of any substance.
It is the number of particles in exactly 12g ( 1 mol) of 12 C.

1 mol 12C atom = 12 g


Avogadro constant, LA 6.0  1023 mol−1
 The relative atomic mass (R.A.M or Ar) in grams of all
elements will contain the same number of particles ( atom,
molecule, …)

1 mol of sulfur 32 g 6.0 1023 atoms
1 mol of magnesium 24 g 6.0 1023 atoms

It is similar to:

1 dozen of oranges 2400g 12 oranges

The mole is the amount of substance, which contains the same number of
particles (atoms or molecules )as there are carbon atoms in 12 g of
12
C.
1. An Avogadro number of particles of any substance.
2. The relative atomic (molecular) mass of a substance expressed in
grams

Mass in grams

m
n=
M Mass of 1 mole of a substance
OR
relative molecular Mass Mr

Molar mass: is the mass of ONE mole of the substance in grams.
It is equal to the relative molecular Mass Mr

IGCSE Chemistry 2021_2022 [Amount of substance - Notes ] AOUDI T.A.

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Examples


 Calculate the number of moles of
(a) 5.40 g of H2O
(b) 50.0 g of CuSO4  5H2O

Ans
m 5.40 g
(a) n = = = 0.3 mol
M 18 g.mol–1

(b) Molar mass of CuSO4  5H2O
= 64  1 + 32.0  1 + 16.0  4 + 5[(1.0  2) + (16.0 1)]
= 250.0 g /mol

m 50 g
n = = = 0.2 mol
M 250 g .mol –1



 Calculate the number of grams (mass) of 0.30 moles of Fe2(SO4)3
Ans:
Molar mass of Fe2(SO4)3 = 2  56 + 3  [32.0  1 + 16.0  4]
= 400 g /mol
m = n  M
= 0.30mol  400 g mol–1
= 120g

Amount of substance & Avogadro's constant

Use Avogadro's Constant LA = 6.0  1023 mol−1

 How many atoms are there in 0.4 mol of iron Fe ?

Ans : N = n  LA
= 0.4 mol  6.0  1023 atoms mol−1
= 2.4  1023 atoms

IGCSE Chemistry 2021_2022 [Amount of substance - Notes ] AOUDI T.A.