Quantitative Practical Skills: Concentration Calculations Using Practical Data (Copy)

Quantitative Practical Skills

Concentration Calculations Using Practical Data

Purpose of Concentration Calculations in ATP

• Concentration calculations are used to:
  • Determine the molarity of a solution from experimental data
• ATP questions test:
  • Accurate use of titration data
  • Correct mole calculations
  • Proper unit conversions
  • Clear stepwise working
• Examiners reward:
  • Use of mean concordant titres
  • Correct stoichiometry
  • Precise significant figures

Core Examiner Rule

• Concentration formula:

  Concentration (mol/dm³) = Moles of solute ÷ Volume of solution (dm³)

• Steps:
  1. Use mean of concordant titres
  2. Convert cm³ → dm³
  3. Calculate moles of titrant
  4. Apply stoichiometric ratio to find moles of analyte
  5. Divide moles by volume of solution in dm³

Step 1: Calculate Volume of Titrant Delivered

• Volume delivered = Final burette reading − Initial burette reading
• Use only concordant titres
• Example:
  • Initial reading: 0.20 cm³
  • Final reading: 23.75 cm³
  • Titre: 23.55 cm³

Step 2: Convert Volume to dm³

• Conversion: Volume (dm³) = Volume (cm³) ÷ 1000
• Example: 23.55 cm³ ÷ 1000 = 0.02355 dm³

Step 3: Calculate Moles of Titrant

• Formula: Moles = Concentration × Volume (dm³)
• Example: If 0.100 mol/dm³ NaOH used:
  Moles NaOH = 0.100 × 0.02355 = 0.002355 mol

Step 4: Apply Stoichiometry to Find Moles of Analyte

• Use balanced chemical equation:

  HCl + Na₂CO₃ → 2 NaCl + H₂O + CO₂

• Mole ratio HCl : Na₂CO₃ = 2 : 1
• Moles Na₂CO₃ = Moles HCl ÷ 2 = 0.002355 ÷ 2 = 0.0011775 mol

Step 5: Calculate Concentration of Analyte

• Concentration (mol/dm³) = Moles ÷ Volume (dm³)
• If solution volume = 25.0 cm³ = 0.0250 dm³:
  Concentration = 0.0011775 ÷ 0.0250 = 0.0471 mol/dm³

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Step 6: Converting Between Mass and Concentration (If Required)

• If question provides mass of solute, calculate moles:

  Moles = Mass ÷ Relative Formula Mass (RFM)

• Then apply concentration formula:

  Concentration = Moles ÷ Volume (dm³)

• Example:
  Mass Na₂CO₃ = 2.64 g, RFM = 106 g/mol, Volume = 0.0500 dm³

  Moles = 2.64 ÷ 106 = 0.0249 mol
  Concentration = 0.0249 ÷ 0.0500 = 0.498 mol/dm³

Step 7: Significant Figures and Units

• Use same significant figures as apparatus allows
• Units: mol/dm³ (not mol/L)
• Round final answer appropriately

Common Examiner Traps

• Using rough titres instead of mean concordant
• Forgetting cm³ → dm³ conversion
• Incorrect mole ratio
• Incorrect significant figures
• Units missing

High-Yield ATP Tips for Concentration Calculations

• Always start with mean concordant titre
• Convert cm³ → dm³ before calculations
• Apply stoichiometry carefully
• Show all steps clearly
• Box final answer with correct units

Core Scientific Principle

• Concentration = Measure of solute per volume of solution
• Accurate practical data and careful calculation ensure:
  • Reliable results
  • Maximum ATP marks
• Mastery of these calculations:
  • Converts experimental data into fully creditable answers