CC1.5 — Chemical equations and calculations (Edexcel 1SC0)

Balancing equations

The law of conservation of mass: atoms are neither created nor destroyed in a chemical reaction. A balanced equation has equal numbers of each atom on both sides.

Example: H₂ + O₂ → H₂O → balance: 2H₂ + O₂ → 2H₂O

Relative formula mass (Mr)

Mr = sum of relative atomic masses of all atoms in the formula.

Example: H₂O: (2 × 1) + 16 = 18. CaCO₃: 40 + 12 + (3 × 16) = 100.

Percentage composition

$$% \text{ composition} = \frac{\text{A}_r \text{ of element} \times \text{number of atoms}}{M_r} \times 100$$

Example: % of Ca in CaCO₃ = (40 / 100) × 100 = 40%.

Conservation of mass in reactions

Total mass of reactants = total mass of products. If a gas is produced and escapes, the mass of the solid appears to decrease — but no atoms are lost.

Concentration

$$\text{concentration (mol/L)} = \frac{\text{moles}}{\text{volume (L)}}$$

Or: concentration (g/L) = mass (g) / volume (L).