In chemistry the amount of a substance is measured in moles. Moles are a unit often abbreviated to mol and are given the symbol n. A mole can be defined as follows:
Mole – the amount of a substance that contains the same number of particles as there are atoms in 12g of carbon-12.
One mole is equal to the Avogadro Constant.
Avogadro Constant (NA) – the number of atoms in 12g of carbon-12.
The Avogadro Constant is approximately equal to 6.02 x 1023 mol-1.
The term particles in the definition of a mole varies depending on what we are talking about. If we are referring to the amount of an atom it refers to the number of atoms. If we are referring to the about of a molecule it refers to the number of molecules.
One Mole of an Atom
The mass of one mole of an atom is equal to its relative atomic mass.
For example, oxygen has a relative atomic mass of 16.0u (1d.p.). This means that one mole of oxygen has a mass of 16g.
Therefore, in 16g of oxygen (1 mole) there are 6.02 x 1023 atoms of oxygen.
One Mole of a Molecule
The mass of one mole of a molecule is equal to its relative molecular mass.
For example, let’s look at water. The formula for water is H2O. We can calculate the relative molecular mass from the relative atomic masses of hydrogen and oxygen:
Relative atomic mass of oxygen: 16u
Relative atomic mass of hydrogen: 1u
The relative molecular mass of H2O is therefore 18u. This means that one mole of water has a mass of 18g.
Therefore, in 18g of water (1 mole) there are 6.02 x 1023 molecules of H2O.
Molar Mass
Molar Mass (M) – the mass per mole of a substance.
The molar mass of a substance can be calculated using the following equation:
Where m is the mass of the substance and n is the amount of the substance (in other words, the number of moles in the substance).
So if we look at oxygen again we know that for 1 mole (n = 1mol) we have 16g of oxygen (m = 16g). Therefore the molar mass of oxygen is:
As you can see this is the equivalent to the relative atomic mass of oxygen. This applies to all atoms. The relative molecular mass of a molecule is equal to the relative molecular mass of that molecule. So for H2O, M = 18gmol-1. You just need to remember the units.
The formula can be useful though if we want to work out the number of moles in a particular mass of a substance or vice versa.
Worked Examples
Example 1
Calculate the mass of 1.5mol of copper sulphate (CuSO4). The relative atomic mass of the elements are given below:
Cu = 63.5u
S = 32.1u
O = 16.0u
Example 2
Calculate the amount, in mol, of 15g of sodium chloride (NaCl). The relative atomic mass of the elements are given below:
Na = 23.0u
Cl = 35.5u
Example 3
What is the mass of nitrogen in 3.5 x 1019 atoms of nitrogen? The relative atomic mass of nitrogen is 14.0u.
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