In a chemical reaction the atoms swap partners. None are made and none are destroyed, so the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass, and it is why a symbol equation must balance.

Yet the reading on a balance can change. That happens when the container is open and a gas is involved:

• Mass goes down when a gas is made and escapes. Heating copper(II) carbonate gives off carbon dioxide, so the solid left is lighter.
• Mass goes up when a gas from the air joins on. Magnesium burning in air takes in oxygen, so the magnesium oxide is heavier than the magnesium was.

1. Marble chips and acid in an open flask: reading falls from 152.40 g to 151.96 g
2. mass of carbon dioxide that escaped = 152.40 − 151.96 = 0.44 g
3. In a sealed flask the reading would not change at all

The mass is not lost. It has moved somewhere the balance cannot weigh it.

The relative formula mass (M_r) of a substance is the sum of the relative atomic masses (A_r) of all the atoms in its formula. A_r values are given in the question or on the periodic table.

Mr = sum of (number of atoms × Ar) for every element in the formula

1. Ca(OH)₂: 1 Ca, 2 O, 2 H (the 2 after the bracket doubles O and H)
2. Mr = 40 + (2 × 16) + (2 × 1) = 74
3. CuSO₄.5H₂O: CuSO₄ = 63.5 + 32 + (4 × 16) = 159.5
4. 5H₂O = 5 × 18 = 90, so Mr = 159.5 + 90 = 249.5

M_r compares masses, so it has no unit. All boards: recall how to work it out (there is no chemistry equation sheet).

The commonest slip: multiplying only the last atom in a bracket. In Al₂(SO₄)₃ the 3 multiplies S and O, giving 3 S and 12 O.

Multiply each M_r by the number in front of its formula and the two sides add to the same total.

1. 2Mg + O₂ → 2MgO
2. 2 × 24 = 48, and 32: reactants total 80
3. 2 × 40 = 80: products total 80
4. So 48 g of magnesium makes 80 g of magnesium oxide, and 4.8 g makes 8.0 g