No atoms are made or lost

The law of conservation of mass states that during a chemical reaction no atoms are created or destroyed. The atoms are just rearranged, so the total mass of the products equals the total mass of the reactants.

That is why a symbol equation must be balanced — the same number of atoms of each element on both sides:

Watch out: multipliers work two ways — a big number in front (the 2 in 2MgO) multiplies the whole formula, while a small subscript (the 2 in O₂) multiplies only the atom before it.

Mass that seems to change

Why mass can seem to change

In an open (non-enclosed) container, mass can appear to rise or fall — but this is only because a gas entered or escaped and was not weighed:

• Mass appears to rise when a gas is taken in — e.g. a metal reacting with oxygen: the oxide is heavier than the metal.
• Mass appears to fall when a gas escapes — e.g. thermal decomposition of a metal carbonate: CO₂ escapes into the air.

Misconception buster: mass is always conserved. In a sealed flask the reading never changes. Seal the same reaction and weigh everything — including the gas — and reactant mass = product mass exactly.

Up, down, or steady?

In each open dish, tap what happens to the reading on the balance.

Relative formula mass, M_r

The relative formula mass (M_r) of a compound is the sum of the relative atomic masses (A_r) of all the atoms in its formula.

Misconception buster: M_r is a ratio of masses — it has no units. (Only when you talk about "the mass of one mole" do grams appear.)

Your turn — find M_r

The mole & the Avogadro constant

Chemical amounts are measured in moles (symbol mol). One mole of any substance contains the same number of particles — the Avogadro constant:

The clever part: the mass of one mole of a substance in grams is numerically equal to its relative formula mass. So one mole of carbon (M_r = 12) is 12 g, and one mole of CO₂ (M_r = 44) is 44 g.

Your turn — mass to moles

Reacting masses from an equation

A balanced equation is a recipe in moles. The big numbers give the mole ratio, which lets you work out the mass of product from the mass of a reactant:

Your turn — reacting masses

Limiting reactants

When two reactants are mixed, one often runs out first. The reactant that is completely used up is the limiting reactant — it limits how much product can form. The other is in excess (some is left over).

Misconception buster: the limiting reactant is the one fully used up — not simply the one with the smaller mass. You must compare moles against the equation's ratio to decide which runs out.

Which one runs out?

Concentration in g/dm³

The concentration of a solution is how much solute is dissolved in a given volume of solution. The first way to measure it is mass per volume:

Your turn — mass of solute

Concentration in mol/dm³

Concentration can also be measured in moles per volume. This links straight to the mole equations:

To go from a g/dm³ concentration to mol/dm³, just divide by the M_r (it is the same "grams → moles" step, per dm³).

Misconception buster: g/dm³ and mol/dm³ are different units. They describe the same solution but are not the same number — convert with the M_r, never assume they are equal.

Your turn — moles in solution

Percentage yield

You rarely collect all the product you'd predict — some is lost on filtering, the reaction may be reversible, or side-reactions occur. The percentage yield compares what you actually got with the theoretical maximum:

No atoms are ever lost in the reaction itself — a low yield means product was lost during the process or the reaction didn't go to completion. (Calculating the theoretical mass from a reactant mass is HT.)

Your turn — percentage yield

Atom economy

The atom economy measures how much of the starting mass ends up as the useful product. High atom economy means less waste — important for sustainability and cost:

Note the difference: % yield is about how much you actually got; atom economy is a fixed property of the equation — it doesn't depend on the lab at all.

Your turn — atom economy

Titration calculations

A titration finds an unknown concentration by reacting two solutions completely. If you know the concentration and volume of one, you can find the other. The key step is moles = concentration × volume:

Your turn — moles from a titration

Volumes of gases

Equal moles of any gas take up equal volumes under the same conditions. At room temperature and pressure (rtp: 20 °C, 1 atm), one mole of any gas occupies 24 dm³:

This works for any gas — 1 mol of hydrogen and 1 mol of carbon dioxide both fill 24 dm³ at rtp, even though they have very different masses.

Your turn — gas volume

Has units, or not?

Tap a quantity, then tap the box it belongs in.

The equations to know

You've covered the whole of AQA 4.3 — from conservation of mass and M_r through moles, reacting masses, concentration, yield, atom economy, titrations and gas volumes. Press Finish to see your score.