Practical chemistry lecture

WAEC 2026 CHEMISTRY PRACTICAL – VOLUMETRIC ANALYSIS

1. Concentration Concepts (mol/dm³ and g/dm³)

Definitions:

• Mol/dm³: Number of moles of solute per 1 dm³ solution.
• g/dm³: Mass of solute per 1 dm³ solution.

Formulas:

Molarity (M) = moles / volume(dm³)
Moles = mass / molar mass
g/dm³ = M × molar mass

Example 1: Concentration in 25 cm³

5g NaOH dissolved in 25 cm³ solution. Find molarity in mol/dm³.

Volume = 25 ÷ 1000 = 0.025 dm³

Moles = 5 ÷ 40 = 0.125 mol

Molarity = 0.125 ÷ 0.025 = 5.0 mol/dm³

Example 2: Convert concentration to 1000 cm³

Concentration in 25 cm³ = 0.5 mol/dm³. Concentration in 1000 cm³ = 0.5 × 1000 ÷ 25 = 20 mol

Example 3: Pure vs Impure substance

Substance weighed = 5g, purity = 80%

Actual pure mass = 5 × 0.80 = 4g

Moles = 4 ÷ 40 = 0.1 mol

Molarity in 100 cm³ (0.1 dm³) solution = 0.1 ÷ 0.1 = 1 mol/dm³

2. Acid-Base Titration – Stepwise Workflow

Only titres differing by ±0.20 cm³ are accepted to calculate the average. Outliers are discarded.

Example: Strong Acid (HCl) vs Strong Base (NaOH)

Titre Type	Volume Acid (cm³)	Volume Base (cm³)	Notes
Rough Titre	19.5	24.0	Estimate endpoint; not used in average
1st Accurate	20.0	25.0	Accepted if difference with 2nd titre ≤ ±0.20
2nd Accurate	19.8	25.0	Accepted if difference with 1st ≤ ±0.20
3rd Accurate	20.3	25.0	Rejected: difference with 2nd = 0.5 > 0.2

Accepted titres: 1st and 2nd (20.0 & 19.8)
Average = (20.0 + 19.8) / 2 = 19.9 cm³

Molarity Calculation: M₁V₁ = M₂V₂ → 0.10 × 25/1000 = M × 19.9/1000 → M ≈ 0.1256 mol/dm³

Weak Acid (CH₃COOH) vs Strong Base (NaOH)

Titre Type	Volume Acid (cm³)	Volume Base (cm³)	Notes
Rough Titre	24.5	29.5	Estimate endpoint
1st Accurate	25.0	30.0	Accepted if difference ≤ ±0.20 cm³
2nd Accurate	24.8	30.0	Accepted (diff = 0.2 cm³)
3rd Accurate	25.3	30.0	Rejected (diff with 2nd = 0.5 cm³ > 0.2)

Average accepted titre: (25.0 + 24.8)/2 = 24.9 cm³

Molarity: M × 24.9/1000 = 0.10 × 30/1000 → M ≈ 0.1205 mol/dm³

3. Indicators in Acid-Base Titration

• Methyl Orange: Acid → Red, Base → Yellow, Neutral → Orange
• Phenolphthalein: Acid → Colorless, Base → Pink, Neutral → Slight Pink
• Methyl Red: Acid → Red, Base → Yellow, Neutral → Orange

Choose indicator according to acid/base type to detect correct endpoint.

4. Percentage Purity & Yield

% purity = (actual / expected) × 100

Example: Expected = 5g, Actual = 4g → % purity = 80%

% yield = (actual / theoretical) ×100

Example: Theoretical yield = 6g, Actual = 4.8g → % yield = 80%

5. Practical Tips

• Always perform a rough titre first.
• Take at least 2–3 accurate titres.
• Only use titres with difference ≤ ±0.20 cm³ for average.
• Read burette at eye level; record to 0.01 cm³.
• Rinse apparatus with solution before titration.
• Choose indicator based on acid/base strength.
• Convert all cm³ volumes to dm³ before calculations.