Concentration
\( c = \frac{n}{V} \)
Unit Warning
Volume MUST be in dm³ (not cm³). To convert: divide cm³ by 1000.
Standard Solutions & Dilutions
Standard Solution: A solution of accurately known concentration.
Dilutions: When you add water, moles stay the same but volume increases. Use:
Titration Calculation
Problem: 25.0 cm³ of 0.100 mol dm⁻³ NaOH is neutralized by 20.0 cm³ of HCl. Calculate the concentration of HCl.
\( NaOH + HCl \rightarrow NaCl + H_2O \) (1:1 ratio)
1. Moles of NaOH: \( n = c \times V = 0.100 \times 0.0250 = 0.00250 \text{ mol} \)
2. Moles of HCl: (1:1 ratio) = 0.00250 mol
3. Concentration of HCl:
\( c = \frac{0.00250}{0.0200} = 0.125 \text{ mol dm}^{-3} \)
Notation and Units
📘 Square Bracket Notation
In IB Chemistry, concentration is often written using square brackets. For example, [HCl] = 0.10 mol dm⁻³ means "the concentration of HCl is 0.10 mol per cubic decimetre".
Converting Between mol dm⁻³ and g dm⁻³
Sometimes concentrations are given in g dm⁻³ rather than mol dm⁻³. To convert:
Concentration (g dm⁻³) = Concentration (mol dm⁻³) × Molar mass (g mol⁻¹)
Example: [NaCl] = 0.50 mol dm⁻³. Molar mass of NaCl = 58.44 g mol⁻¹. Concentration = 0.50 × 58.44 = 29.2 g dm⁻³.
⚠️ Examiner Tip: ppm
Very dilute solutions (e.g. Pollutants in water) may use parts per million (ppm). For aqueous solutions: 1 ppm = 1 mg dm⁻³. This assumes the density of water is approximately 1 g cm⁻³.