The Mole & Reacting Masses (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about the mole, the Avogadro constant and reacting masses.

The mole converts a mass you can weigh into a number of particles, and reacting masses applies that same conversion on both sides of a balanced equation.

The Avogadro constant fixes the number of particles in one mole, and the mass of one mole of a substance is its relative atomic mass or relative formula mass expressed in grams. Those two facts together are what allow a mass in grams to be converted into a number of particles. Reacting masses uses that same conversion, because a balanced equation gives the ratio of moles in which substances react, so a mass of one substance can be converted into a mass of another.

The video is in three steps: what a mole is and what the Avogadro constant counts, then the two equations that move between mass, moles and particles, then reacting masses, where the mole ratio in a balanced equation turns one mass into another.

A mole is the amount of substance that contains the same number of elementary entities, for example atoms or ions, as there are atoms in exactly 12 grams of carbon-12. The Avogadro constant, L, is the number of particles in one mole of a substance, and its value is six point zero two times ten to the twenty-three per mole.

It applies to atoms, molecules, ions and electrons, so a mole is always a count of whichever particle is being named.

One mole of an element has a mass equal to its relative atomic mass expressed in grams, and one mole of a compound has a mass equal to its relative formula mass expressed in grams. One mole of carbon has a mass of 12 grams and contains six point zero two times ten to the twenty-three carbon atoms. One mole of water has a mass of 18 grams and contains that same number of water molecules. One mole of sodium chloride has a mass of 58.5 grams, which is the sum of the relative atomic masses of sodium and chlorine.

A mass in grams and a number of particles are therefore two ways of stating the same amount of substance.

Two equations move between those quantities. The number of moles is the mass of the substance in grams divided by the molar mass in grams per mole. Molar mass has the same numerical value as the relative atomic or relative formula mass but these have no units. The number of particles is the number of moles multiplied by the Avogadro constant.

The formula triangle holds both. Cover the quantity you want to find and follow the directions in the triangle.

It is important to be clear about the type of particle being referred to. One mole of calcium fluoride contains one mole of calcium fluoride formula units, but one mole of calcium two plus ions and two moles of fluoride ions.

The same care applies to molecules. One mole of hydrogen gas contains six point zero two times ten to the twenty-three hydrogen molecules, but because there are two hydrogen atoms in every molecule, it contains twice that many hydrogen atoms.

These are the conversions that reacting masses calculations are built from.

The masses of reactants are useful to determine how much of the reactants exactly react with each other, which prevents waste.

To calculate the reacting masses, the balanced chemical equation is required. The equation shows the ratio of moles of all the reactants and products, and that ratio is called the stoichiometry of the equation. The stoichiometry can be found if the exact amounts of reactants and products formed are known, and those amounts come from the same mass divided by molar mass equation.

To find the mass of the product formed, three pieces of information are needed: the mass of the reactants, the molar mass of the reactants, and the balanced equation. Each mass is converted to moles using its molar mass, so the mole conversion is used on both sides of the equation.

Gas volumes can also be used to deduce the stoichiometry of a reaction. In the combustion of 50 centimetres cubed of propane reacting with 250 centimetres cubed of oxygen, 150 centimetres cubed of carbon dioxide is formed, which suggests a propane to oxygen to carbon dioxide ratio of one to five to three. The balanced equation is one propane molecule plus five oxygen molecules going to three carbon dioxide molecules plus four water molecules.

One mole contains six point zero two times ten to the twenty-three particles, and the mass of one mole is the relative atomic mass or relative formula mass expressed in grams.

The number of moles is calculated by dividing mass by molar mass, and the number of particles is moles multiplied by the Avogadro constant.

A balanced equation gives the ratio of moles in which substances react.

The mole links a mass that can be weighed to a number of particles, and a balanced equation carries that link across a reaction.