What is a mole?

A mole is a unit of amount of substance. One mole is the number of atoms of carbon in 12g of the carbon-12 isotope(when 12 is the relative atomic mass) , which is 6.02 x 10²³ . This means that 0.5 moles of carbon will have 3.01 x 10²³ atoms and will have a mass of 6g whereas 2 moles will have 1.24 x 10²⁴ atoms and a mass of 24g. But the main purpose of moles is to compare amounts of different substances so we can use and predict correct accounts in experiments. One mole of other substances e.g calcium which has a relative atomic mass of 40, have different masses. This is because each calcium atom is heavier than each carbon atom (40 compared to 12). Therefore the same number of atoms of calcium which make up a mole (which is 6.02 x 10²³)will actually have a mass of 40g. As you can see the mass of one mole of an element is equal to it's relative atomic mass in grams; this can be a useful way to think about it. We can therefore express a mole with the equation n=m/Mr, where n is number of moles, m is mass in gram and Mr is relative atomic mass. You may be asked to calculate the number of moles of a substance in a given mass. For example calculate the number of moles in 100g of iron. On the periodic table you would find the relative atomic mass (Mr) of iron which is 56 and then 100 will be your value of m. 100/56 will give you a value of 1.786 moles of iron.