The core relationships
- Number of moles = mass ÷ molar mass
- Number of particles = moles × 6.022 × 10²³ (Avogadro's number)
- Molar volume of any ideal gas at STP = 22.4 L
- Molarity (M) = moles of solute ÷ litres of solution
Worked example
What is the mass of 2 moles of CaCO₃?
Molar mass of CaCO₃ = 40 + 12 + (16 × 3) = 100 g/mol Mass = moles × molar mass = 2 × 100 = 200 g.
pH shortcuts
- pH = −log[H⁺]
- If [H⁺] = 1 × 10⁻ⁿ, then pH = n.
- Neutral at 25 °C → pH 7. Acidic < 7 < basic.
Must-know molar masses
| Compound | Molar mass (g/mol) |
|---|---|
| H₂O | 18 |
| CO₂ | 44 |
| NaCl | 58.5 |
| NaOH | 40 |
| CaCO₃ | 100 |
| H₂SO₄ | 98 |
Most MECEE chemistry numericals reduce to "convert to moles first." Master that one step and the rest follows.