The Mole Unit (DP IB Chemistry): Revision Note
The Mole
The Avogadro constant (NA or L) is the number of particles equivalent to the relative atomic mass (Ar) or molecular mass (Mr) of a substance in grams
The Avogadro constant applies to atoms, molecules and ions
The value of the Avogadro constant is 6.02 x 1023 mol-1
The mass of a substance with this number of particles is called the molar mass
One mole of a substance contains the same number of fundamental units as there are atoms in exactly 12.00 g of 12C
6.02 x 1023 atoms of 12C has mass of exactly 12.00 g
1 mole of H2O = (2 x 1.01 + 16.00) = 18.02 g
Worked Example
Determine the number of atoms, molecules and the relative mass of 1 mole of:
Na
H2
NaCl
Answer 1:
The Ar of Na is 22.99
1 mol of Na has a mass of 22.99 g mol-1
1 mol of Na will contain 6.02 x 1023 atoms of Na
Answer 2:
The Ar of H is 1.01
There are 2 H atoms in H2 so the mass of 1 mol of H2 is (2 x 1.01) 2.02 g mol-1
1 mol of H2 will contain 6.02 x 1023 molecules of H2
As there are 2 H atoms in each molecule of H2, 1 mol of H2 molecules will contain 1.204 x 1024 H atoms
Answer 3:
The Ar of Na and Cl are 22.99 and 35.45 respectively
1 mol of NaCl has a mass of (22.99 + 35.45) 58.44 g mol-1
1 mol of NaCl will contain 6.02 x 1023 formula units of NaCl
As there is both an Na and a Cl atom in NaCl, 1 mol of NaCl will contain 1.204 x 1024 atoms in total
Summary:
Na is a monoatomic element:
Atoms = 6.02 × 1023
Ar = 22.99
H2 is a diatomic molecule:
Molecules = 6.02 × 1023
Atoms = 2 × 6.02 × 1023 = 1.204 x 1024
Mr = 2.02
NaCl consists of Na⁺ and Cl⁻
Formula units = 6.02 × 1023
Ions = 2 × 6.02 × 1023 = 1.204 x 1024
Mr = 58.44
Relative Atomic Mass
Relative atomic mass, Ar
The relative atomic mass (Ar) of an element is the weighted average mass of one atom compared to one twelfth the mass of a carbon-12 atom
The relative atomic mass is determined by using the weighted average mass of the isotopes of a particular element
The Ar has no units as it is a ratio and the units cancel each other out
Calculating Mr
H2:
Atoms present: 2 x H
Mr = (2 x 1.01) = 2.02
H2O:
Atoms present: (2 x H) + (1 x O)
Mr = (2 x 1.01) + 16.00 = 18.02
K2CO3
Atoms present: (2 x K + (1 x C) + (3 x O)
Mr = (2 × 39.10) + 12.01 + (3 × 16.00) = 138.21
Ca(OH)2
Atoms present: (1 × Ca) + (2 × O) + (2 × H)
Mr = 40.08 + (2 × 16.00) + (2 × 1.01) = 74.10
(NH4)2SO4
Atoms present: (2 × N) + (8 × H) + (1 × S) + (4 × O)
Mr = (2 × 14.01) + (8 × 1.01) + 32.07 + (4 × 16.00) = 132.17
Relative formula mass, Mr
The relative formula mass (Mr) is used for compounds containing ions
It is calculated in the same way as relative molecular mass
In the table above, the Mr for potassium carbonate, calcium hydroxide and ammonium sulfate are relative formula masses
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