What is a mole and why do we use it? (chemistry)

The mole is a concept that you'll see everywhere in chemistry, from GCSE through A level. Although this is the case, it can be very easy to lose sight of what it actually is and why we use it, as your teacher may explain it briefly and then never do so again. It has an exact definition, which you probably will only need to learn exactly if you're an A level student: the mass of substance that contains the same number of particles as there are atoms in exactly 12.000 g of ¹²C. This is a complicated way of saying that the mole is a standard number of particles across any compound or element. For example, one mole of hydrogen will contain the same number of particles as one mole of lead, despite them weighing very different amounts. The mole is especially useful when writing chemical equations, because it lets us compare the ratio in which particles react rather than ratio of masses. For example: C + O2 = CO2 shows that one mole of carbon reacts with one mole of oxygen to form one mole of carbon dioxide. This is much more helpful than knowing 1 gram of carbon reacts with 2.7 grams of oxygen to form one mole of carbon dioxide. In case you are asked how many particles or 'fundamental units' are in a compound, you should be familiar with the Avogadro Constant: 6.02 x 10^23 mol^-1. This means that there are that many particles in one mole of substance! If you multiple the number of moles of substance you have by the Avogadro constant, you'll find out how many particles you have.