Relative Formula Mass & Balanced Equations (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about relative masses and balanced equations. The two are covered together because both run off the formula of a compound: the relative formula mass is the relative atomic masses of every atom in that formula added together, and balancing an equation keeps the number of each of those atoms the same on both sides.

Relative atomic mass and relative molecular mass are ratios that compare the mass of an atom or a molecule to a fixed standard. A balanced equation is a symbol equation in which the number of each type of atom is the same on both sides. The formula of the compound is what joins them: the relative molecular mass is the relative atomic masses of every atom in the formula unit added together, and balancing an equation means counting those same atoms and making them equal on each side, without changing any of the formulae.

We will cover the standard first, what the masses are measured against and what A r and M r each mean. Then where a formula comes from, which for an ionic compound is the charges on its ions. Then balancing the equation itself, and what an ionic equation leaves out.

A single atom is far too small to weigh directly, so atomic masses are defined against a standard atom instead. That standard is the unified atomic mass unit, and it is one twelfth of the mass of a carbon-12 isotope.

Relative atomic mass, A r, is the ratio of the average mass of an element's atoms to that unit. It is an average because most elements exist as isotopes, atoms of the same element with different numbers of neutrons, and each isotope has its own relative isotopic mass. Multiply each isotope's mass by its percentage abundance, add those together and divide by one hundred, and that weighted average is the formula on screen. Because A r is a ratio, the units cancel, so it has none.

Relative molecular mass, M r, does the same job for a molecule: the ratio of the weighted average mass of a molecule to the same unit, and it has no units either. You find it by adding up the relative atomic masses of every atom in one molecule, using the simplest formula for the compound, the formula unit. Silicon dioxide has a giant covalent structure, but its formula unit is SiO2. When the compound contains ions, the same number is called the relative formula mass: same units, same calculation.

M r is therefore a count of the atoms in the compound's formula, each one weighted by its relative atomic mass.

For an ionic compound, that formula comes from the charges. An ionic compound is a metal bonded to a non-metal, and it is electrically neutral, so the positive charges have to equal the negative ones. This chart is where those charges come from. All metals form positive ions: Group 1 is one plus, Group 2 is two plus, Group 3 is three plus. There are a few non-metal positive ions as well, ammonium and hydrogen among them.

Non-metals in Groups 15 to 17 form negative ions and take the suffix "ide". Group 17 gains one electron, so forms a one minus ion; Group 16 gains two; Group 15 gains three.

The transition elements are different, because their charge can vary, and that is what the Roman numerals are for: copper two oxide has a copper ion with a two plus charge, copper one nitrate has a copper ion with a charge of one plus. Some negative ions are polyatomic, made of more than one type of atom, and where a formula needs more than one of them, it goes in brackets.

Balancing the charges is what fixes the formula, and it is that formula whose atoms are counted for the relative formula mass and balanced in the equation.

A symbol equation describes a reaction using chemical symbols, showing the number and type of each atom in the reactants and the products; a word equation does the same thing in words.

Atoms cannot be created or destroyed in a chemical reaction, so the number of each atom has to be the same on both sides. That is what balancing means, and it is why the formulae are settled before you start: you do not change any of them, you put the balancing numbers in front of them.

The method is to write the formulae of the reactants and the products, count the atoms in each, then balance them one at a time. In combustion reactions of organic compounds, balance the carbon first, then the hydrogen, then the oxygen. Then add the state symbols: s for solid, l for liquid, g for gas, aq for aqueous.

Ionic equations go one step further. In aqueous solution, ionic compounds dissociate into their ions, and often only some of those ions take part in the reaction. The ones that do not are spectator ions, and an ionic equation shows only the particles that do take part.

Relative atomic mass is not the mass number: A r is a weighted average across all the isotopes, whereas mass number applies to a single isotope. And A r and M r are dimensionless ratios, so neither of them carries units.

A r and M r are both ratios measured against one twelfth of the mass of a carbon-12 atom. M r is the relative atomic masses of every atom in the formula unit added together, and it is called the relative formula mass when the compound is ionic. For an ionic compound, the charges on the ions give you that formula, and balancing an equation keeps the number of each of those atoms the same on both sides. It is the same formula doing both jobs.