⚖️ Molar Master Level 4 · HL

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Back Titration

  • Back titration is used when the analyte is insoluble, a slow reaction, or when a direct titration has no sharp endpoint
  • The analyte reacts with a known excess of reagent. The excess is then titrated to determine how much reacted
  • Key equation: n(reacted) = n(added) − n(excess)
  • Common applications: purity of CaCO₃ in antacids/eggshells, aspirin purity

5-Step Method

  • Step 1: Calculate moles of excess reagent from titration: n = c × V
  • Step 2: Calculate original moles of reagent added: n = c × V
  • Step 3: Moles that reacted = original − excess
  • Step 4: Use stoichiometry to find moles of analyte
  • Step 5: Convert to mass (m = n × M) or purity (%)

IB Exam Strategies

  • Common mistake: using titration volume/concentration directly — remember the titration measures the EXCESS, not the analyte
  • Units: always convert cm³ → dm³ before using c = n/V
  • Stoichiometry: check the mole ratio carefully. CaCO₃ + 2HCl → the ratio is 1:2
  • Sig figs: match the least number of significant figures from the given data