Key Learning Objectives
- Carry out and describe the step-by-step acid-base titration method using pipettes, burettes, and indicators.
- Calculate mean concordant titres within 0.10 cm³ of each other, discarding rough trials.
- Calculate unknown solution concentrations using n = c × V and balanced chemical equations.
Topic Summary & Core Notes
Master the required acid-base titration practical. Read burettes to 0.05 cm³, identify concordant titres, use indicators correctly, and solve Concentration = Moles ÷ Volume calculations.
Key Rules, Formulas & Definitions
Solution Concentration
Moles (n) = Concentration (mol/dm³) × Volume (dm³) [n = c × V]
Volume Conversion
Volume in dm³ = Volume in cm³ ÷ 1,000
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Selecting Concordant Titres
Question: A student records four titration titres: Rough = 24.50 cm³, Trial 1 = 23.40 cm³, Trial 2 = 23.50 cm³, Trial 3 = 23.85 cm³. Calculate the mean titre.
Step-by-Step Solution:
- Step 1: Identify concordant trials: Concordant results are within 0.10 cm³ of each other: 23.40 cm³ and 23.50 cm³ (difference = 0.10 cm³). Discard rough (24.50) and trial 3 (23.85).
- Step 2: Calculate average: Mean = (23.40 + 23.50) ÷ 2 = 23.45 cm³.
Final Answer: 23.45 cm³
Senior Examiner Insight: Never include rough titres or non-concordant trials (>0.10 cm³ difference) in the mean calculation.
Worked Example 2
Worked Example 2: Calculating Acid Concentration
Question: 25.0 cm³ of 0.100 mol/dm³ NaOH neutralises 20.0 cm³ of HCl: NaOH + HCl → NaCl + H2O. Calculate the concentration of the HCl solution.
Step-by-Step Solution:
- Step 1: Convert volumes to dm³: V(NaOH) = 25.0 ÷ 1,000 = 0.025 dm³. V(HCl) = 20.0 ÷ 1,000 = 0.020 dm³.
- Step 2: Moles of NaOH: n = c × V = 0.100 × 0.025 = 0.0025 mol.
- Step 3: Moles of HCl: 1:1 stoichiometric ratio → n(HCl) = 0.0025 mol.
- Step 4: Concentration of HCl: c = n ÷ V = 0.0025 ÷ 0.020 = 0.125 mol/dm³.
Final Answer: 0.125 mol/dm³
Senior Examiner Insight: Always convert cm³ to dm³ before multiplying by concentration.
Common Pitfalls & Examiner Warnings
Common Mistake: Using Universal Indicator in a titration
Universal indicator changes colour gradually over a wide range; single-colour indicators (phenolphthalein or methyl orange) give a sharp endpoint.
Common Mistake: Including the rough trial in the mean titre
The rough trial is a coarse estimate and must be excluded from calculations.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What is the colour change of phenolphthalein indicator when adding acid to an alkali?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Pink to Colourless
Detailed Explanation: Phenolphthalein is bright pink in alkaline solutions and turns colourless at the neutralisation endpoint.
Mark Scheme & Scoring Points: 1 mark for Pink to Colourless.
Question 2 of 3
What maximum difference between repeat titres qualifies them as 'concordant'?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) 0.10 cm³
Detailed Explanation: Exam boards define concordancy as readings within ±0.10 cm³ of each other.
Mark Scheme & Scoring Points: 1 mark for 0.10 cm³.
Question 3 of 3
Why should the conical flask be swirled continuously during a titration?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) To ensure thorough and immediate mixing of reactants so the endpoint is exact
Detailed Explanation: Continuous swirling ensures the acid and alkali mix immediately, preventing premature or delayed colour changes.
Mark Scheme & Scoring Points: 1 mark for ensuring thorough mixing of reactants.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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