CC1.6 — Moles and calculations (Edexcel 1SC0, Higher tier)

The mole

One mole of any substance contains 6.02 × 10²³ particles (Avogadro's number).

$$\text{moles} = \frac{\text{mass (g)}}{M_r}$$

Mass calculations

1. Write the balanced equation.
2. Work out moles of the known substance.
3. Use the molar ratio from the equation.
4. Calculate mass of the unknown.

Example: 2H₂ + O₂ → 2H₂O. If 4 g of H₂ reacts: moles H₂ = 4/2 = 2 mol. Ratio H₂:H₂O = 1:1. Moles H₂O = 2 mol. Mass H₂O = 2 × 18 = 36 g.

Limiting reactants

The limiting reactant is the one that runs out first — it determines the maximum yield. The other reactant is in excess.

To find which is limiting: calculate moles of each reactant; compare the mole ratio from the equation.

Concentration

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

$$\text{moles} = \text{concentration} \times \text{volume (L)}$$