Conservation of mass and balanced equations

Conservation of mass

In a closed chemical reaction the total mass of reactants equals the total mass of products. Atoms are not created or destroyed — they are rearranged.

If mass appears to change in a non-sealed reaction:

• Apparent decrease — a gas has escaped (e.g. CO2 from a metal carbonate).
• Apparent increase — a gas has been absorbed (e.g. magnesium burning in air gains oxygen).

Balanced equations

A balanced symbol equation has the same number of each type of atom on both sides.

Example: balance Mg + O2 to give MgO.

• Left: 1 Mg, 2 O. Right: 1 Mg, 1 O. Unbalanced.
• Multiply MgO by 2: 2 Mg, 2 O on the right. Now multiply Mg by 2.
• Final: 2Mg + O2 to give 2MgO. Both sides have 2 Mg and 2 O.

Relative formula mass (Mr)

Sum the relative atomic masses (Ar) of all atoms in the formula.

Example: Mr(CaCO3) = 40 + 12 + (3 × 16) = 100.

Mr is dimensionless. Use it for percentage composition and reacting masses.

Reacting masses

Once you have a balanced equation, scale by the ratio of Mr values.

Example: how much MgO from 6 g of Mg in 2Mg + O2 to give 2MgO?

• Mr(Mg) = 24, Mr(MgO) = 24 + 16 = 40.
• Ratio: 2 × 24 = 48 g Mg gives 2 × 40 = 80 g MgO.
• Per gram of Mg: 80 / 48 = 5/3.
• 6 g Mg gives 6 × 5/3 = 10 g MgO.

WJEC exam tip

When balancing, only change coefficients in front of formulae — never change subscripts. Changing H2O to H2O2 changes the substance entirely.