Chemical reactions: equations and masses

Conservation of mass

In any chemical reaction, the total mass of the reactants equals the total mass of the products. No atoms are created or destroyed — they are only rearranged.

If a reaction in an open container appears to lose mass, it is because a gas has escaped (e.g. CO₂ from heating CaCO₃). If it appears to gain mass, a gas from the air has been absorbed (e.g. O₂ during combustion of magnesium).

Balancing equations

A balanced equation has the same number of each atom on both sides. Use coefficients only — never change subscripts inside formulae.

Example: combustion of methane

CH₄ + 2O₂ → CO₂ + 2H₂O

Atoms — left: 1 C, 4 H, 4 O. Right: 1 C, 4 H, 4 O. Balanced.

Relative formula mass (Mr)

Add the relative atomic masses of every atom in a formula.

• H₂O: 2(1) + 16 = 18
• CaCO₃: 40 + 12 + 3(16) = 100
• (NH₄)₂SO₄: 2(14 + 4) + 32 + 4(16) = 132

Percentage by mass

% mass of element = (Ar × number of atoms in formula) / Mr × 100

Worked example — % nitrogen in NH₄NO₃ (ammonium nitrate fertiliser): Mr(NH₄NO₃) = 14 + 4 + 14 + 48 = 80 N atoms total = 2, mass of N = 28 % N = 28 / 80 × 100 = 35%

This calculation is the basis for comparing fertiliser quality.

State symbols

(s) solid · (l) liquid · (g) gas · (aq) dissolved in water (aqueous solution).

OCR PAG C2 — investigating mass changes

Heating a metal carbonate in a crucible: mass decreases as CO₂ escapes. Mass conservation is shown by including the released gas in the calculation.

OCR exam tip

When balancing, change coefficients in front of formulae only. Subscripts inside formulae are part of the chemical identity. Changing H₂O to H₂O₂ creates a different compound (hydrogen peroxide).