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This result is so important that scientists decided to use a special unit of measurement to define this quantity: the mole or 'mol'. A mole is defined as being an amount of a substance which contains the same number of particles as there are atoms in 12 g of carbon. In the examples that were used earlier, 24.31 g magnesium is one mole of magnesium, while 40.08 g of calcium is one mole of calcium. A mole of any substance always contains the same number of particles.
The mole (abbreviation 'n') is the SI (Standard International) unit for 'amount of substance'. It is defined as an amount of substance that contains the same number of particles (atoms, molecules or other particle units) as there are atoms in 12 g carbon.
In one mole of any substance, there are 6.023 x particles. This is known as Avogadro's number .
The number of particles in a mole, equal to 6.023 x . It is also sometimes referred to as the number of atoms in 12 g of carbon-12.
Element | Relative atomic mass (u) | Sample mass (g) | Number of moles in the sample |
Hydrogen | 1.01 | 1.01 | |
Magnesium | 24.31 | 24.31 | |
Carbon | 12.01 | 24.02 | |
Chlorine | 35.45 | 70.9 | |
Nitrogen | 42.08 |
Molar mass (M) is the mass of 1 mole of a chemical substance. The unit for molar mass is grams per mole or g.mol .
Refer to [link] . You will remember that when the mass, in grams, of an element is equal to its relative atomic mass, the sample contains one mole of that element. This mass is called the molar mass of that element.
It is worth remembering the following: On the Periodic Table, the relative atomic mass that is shown can be interpreted in two ways.
Element | Relative atomic mass (u) | Molar mass (g.mol ) | Mass of one mole of the element (g) |
Magnesium | 24.31 | 24.31 | 24.31 |
Lithium | 6.94 | 6.94 | 6.94 |
Oxygen | 16 | 16 | 16 |
Nitrogen | 14.01 | 14.01 | 14.01 |
Iron | 55.85 | 55.85 | 55.85 |
Calculate the number of moles of iron (Fe) in a 111.7 g sample.
If we look at the periodic table, we see that the molar mass of iron is 55.85 g.mol . This means that 1 mole of iron will have a mass of 55.85 g.
If 1 mole of iron has a mass of 55.85 g, then: the number of moles of iron in 111.7 g must be:
There are 2 moles of iron in the sample.
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