What Is A Mole?

All things avogadro, moles and amount are a crucial element (pardon the pun) to A-Level, and even GCSE, chemistry. However, students often come up to A-Level Chemistry unsure of exactly what a `mole` means, making it harder to understand exam questions which almost always need us to work with quantities and molecular masses.

Simply put, a mole is the number of atoms in 12g of carbon-12. Therefore, a mole of any substance contains the same number of atoms as there are in 12g of carbon-12, and can be calculated using the simple equation: M/Ar, or mass/relative atomic mass. From years of experiments, we now know that one mole of anything contains 6.02x10²³ molecules (or atoms/particles, however you like to remember it): this is known as Avogadro`s constant.

Scientists in the 1800s realised that atoms react in whole ratios. For example, you can`t have 1/5<sup>th</sup> of an atom reacting with another, this just doesn`t make sense, but 1 atom could react with 2 or 3 other atoms. They wanted a way of counting these molecules this is where carbon-12 and Avogadro`s constant came in use.

Later down the line, they finally discovered relative atomic masses of the elements (this is the average masses of the isotopes of an element, but this is another topic for a different day), and now knew that, say, an oxygem atom`s mass is 16 times the mass of a hydrogen atom. Henceforth, if you measured out 16g of oxygen and 1g of hydrogen, both of the samples would have the same number of atoms.

During the 1960s, scientists agreed to use carbon-12 as the standard for all substances, since carbon-12 has the same number of protons and neutrons and isn`t as low a mass number to work with as hydrogen, whose relative atomic mass is 1, meaning it is fairly accurate to estimate the number of particles in 12g of carbon-12. Thus, from this all work, we now know that if we had 16g of oxygen, the number of particles in this sample would be the same as in 12g of carbon-12, since oxygen is 16/12 the mass of an atom of carbon-12.