Moles and molar mass

Quantitative aspects of chemical change

An equation for a chemical reaction can provide us with a lot of useful information. It tells us what the reactants and the products are in the reaction, and it also tells us the ratio in which the reactants combine to form products. Look at the equation below:

$\mathrm{Fe}+\mathrm{S}\to \mathrm{FeS}$

In this reaction, every atom of iron ( $\mathrm{Fe}$ ) will react with a single atom of sulphur ( $\mathrm{S}$ ) to form one molecule of iron sulphide ( $\mathrm{FeS}$ ). However, what the equation doesn't tell us, is the quantities or the amount of each substance that is involved. You may for example be given a small sample of iron for the reaction. How will you know how many atoms of iron are in this sample? And how many atoms of sulphur will you need for the reaction to use up all the iron you have? Is there a way of knowing what mass of iron sulphide will be produced at the end of the reaction? These are all very important questions, especially when the reaction is an industrial one, where it is important to know the quantities of reactants that are needed, and the quantity of product that will be formed. This chapter will look at how to quantify the changes that take place in chemical reactions.

The mole

Sometimes it is important to know exactly how many particles (e.g. atoms or molecules) are in a sample of a substance, or what quantity of a substance is needed for a chemical reaction to take place.

The amount of substance is so important in chemistry that it is given it's own name, which is the mole.

Mole

The mole (abbreviation 'n') is the SI (Standard International) unit for 'amount of substance'.

Now that we know what a mole is, we can relate it to something that we know already. This is the relative atomic mass. For example, if we have a sample containing 1g of hydrogen then we have 1 mole of hydrogen, since the relative atomic mass of hydrogen is 1u.

We can build up to the idea of Avogadro's number. For example, if you have 12 eggs then you have a dozen eggs. After this number we get a gross of eggs, which is 144 eggs. Finally if we wanted one mole of eggs this would be $6,022\times {10}^{23}$ . That is a lot of eggs!

In one mole of any substance, there are $6,022\times {10}^{23}$ particles. When we talk about the mole, we should always say what the particles are. The particles can be atoms, molecules, electrons, or almost anything else.

Avogadro's number

The number of particles in a mole, equal to $6,022\times {10}^{23}$ .

If we were to write out Avogadro's number then it would look like: $602200000000000000000000$ . This is a very large number. If we had this number of cold drink cans, then we could cover the surface of the earth to a depth of over $300\mathrm{km}$ ! If you could count atoms at a rate of 10 million per second, then it would take you 2 billion years to count the atoms in one mole!