Moles and molar mass  (Page 2/4)

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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.

Mole

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 ${10}^{23}$ particles. This is known as Avogadro's number .

The number of particles in a mole, equal to 6.023 x ${10}^{23}$ . It is also sometimes referred to as the number of atoms in 12 g of carbon-12.

Interesting fact

The original hypothesis that was proposed by Amadeo Avogadro was that 'equal volumes of gases, at the same temperature and pressure, contain the same number of molecules' . His ideas were not accepted by the scientific community and it was only four years after his death, that his original hypothesis was accepted and that it became known as 'Avogadro's Law'. In honour of his contribution to science, the number of particles in one mole was named Avogadro's number .

Moles and mass

1. Complete the following table:
 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
2. How many atoms are there in...
1. 1 mole of a substance
2. 2 moles of calcium
3. 5 moles of phosphorus
4. 24.31 g of magnesium
5. 24.02 g of carbon

Molar mass

Molar mass

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 ${}^{-1}$ .

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.

1. The mass of a single, average atom of that element relative to the mass of an atom of carbon.
2. The mass of one mole of the element . This second use is the molar mass of the element.
 Element Relative atomic mass (u) Molar mass (g.mol ${}^{-1}$ ) 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.

1. If we look at the periodic table, we see that the molar mass of iron is 55.85 g.mol ${}^{-1}$ . This means that 1 mole of iron will have a mass of 55.85 g.

2. 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:

$\frac{111.7g}{55.85g.mo{l}^{-1}}=2mol$

There are 2 moles of iron in the sample.

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