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 or molecular mass 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
If you had 6.02 x 1023 atoms of carbon-12 in your hand, you would have a mass of exactly 12.00 g
One mole of water would have a mass of (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 relative atomic mass of Na is 22.99
Therefore, 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 (Avogadro’s constant)
Answer 2:
The relative atomic mass of H is 1.01
Since there are 2 H atoms in H2, 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
However, since 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 relative atomic masses of Na and Cl are 22.99 and 35.45 respectively
Therefore, 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
Since 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:
1 mole of | Number of atoms | Number of molecules/ formula units | Relative mass |
|---|---|---|---|
Na | 6.02 x 1023 | - | 22.99 |
H2 | 1.204 x 1024 | 6.02 x 1023 | 2.02 |
NaCI | 1.204 x 1024 | 6.02 x 1023 | 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
Table of Relative Molecular Mass Calculations
Substance | Atoms present | Mr |
|---|---|---|
Hydrogen (H2) | 2 x H | ( 2 x 1.01) = 2.02 |
Water (H2O) | (2 x H) + (1 x O) | (2 x 1.01) + 16.00 = 18.02 |
Potassium Carbonate (K2CO3) | (2 x K) + (1 x C) +(3 x O) | (2 x 39.10) + 12.01+ ( 3 x 16.00) = 138.21 |
Calcium hydroxide (Ca(OH)2) | (1 x Ca) + (2 x O) +(2 x H) | 40.08 x ( 2 x 16.00) + (2 x 1.01) = 74.10 |
Ammonium Sulfate (NH4)2SO4) | (2 x N) + (8 x H) +(1 x S) + (4 x O) | (2 x 14.01) + ( 8 x 1.01) + 32.07 + ( 4 x 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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