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7.1.3Additional lessons

Applying Techniques in Chemistry — IB Chemistry HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 160 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 6Quantitative Techniques

These methods turn observations into numbers that can be processed.

Titration
  • Titration A solution of known concentration is added from a burette until an indicator shows the end point,
    • which allows the concentration of the other solution to be found.
Back titration
  • Back titration An excess of reagent is added, and the leftover is titrated, which suits insoluble solids or slow reactions.
Gravimetric analysis
  • Gravimetric analysis A product is isolated, dried and weighed to constant mass so that its amount can be calculated.
Calorimetry
  • Calorimetry The temperature change of a known mass of water or solution is measured to find the energy transferred by a reaction.

Section 2 of 6Separation and Purification

Separation and Purification
Filtration
  • Filtration An insoluble solid is separated from a liquid by passing the mixture through filter paper in a funnel.
Distillation
  • Distillation Liquids with different boiling points are separated by heating, condensing the vapour and collecting each fraction in turn.
Recrystallisation
  • Recrystallisation A solid is dissolved in the minimum amount of hot solvent and crystals form as it cools, leaving impurities in solution.
Chromatography
  • Chromatography Substances move at different rates through a stationary phase, so the mixture separates according to attraction to each phase.

Section 3 of 6Setting Up a Reaction Safely

1
Heating under reflux uses a condenser held vertically so that vapours return to the flask and none of the volatile liquid escapes.
2
Anti-bumping granules are added to the flask so that liquid boils smoothly instead of erupting.
3
A flame is avoided for flammable liquids, and an electric heating mantle or water bath is used instead.
4
For distillation the thermometer bulb sits level with the side arm so that it records the vapour temperature.
5
The apparatus is never sealed at both ends while heating, which prevents a pressure build-up.

Section 4 of 6Choosing a Technique

Match to the aim
  • Match to the aim Use titration for concentration, distillation for liquids,
    • and filtration for insoluble solids,
    • because each technique answers a particular question.
Purity check
  • Purity check A melting point or boiling point that is sharp and close to the literature value suggests that the sample is pure.
Yield
  • Yield Losses on transfer, filtration and drying lower the yield, so the percentage yield rarely reaches 100 %.
Risk
  • Risk Assess the hazards of each chemical and technique, and select the control measures such as fume cupboards, eye protection and gloves.

Section 5 of 6Worked Example

Question

  • 25.00 cm³ of hydrochloric acid is titrated with 0.100 mol dm⁻³ sodium hydroxide and the mean titre is 22.50 cm³.
  • Calculate the concentration of the acid.

  1. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l), so the ratio is 1 : 1.
  2. n(NaOH) = 0.100 × 22.50 ÷ 1000 = 2.250 × 10⁻³ mol.
  3. n(HCl) = 2.250 × 10⁻³ mol, and concentration = 2.250 × 10⁻³ ÷ 0.02500 = 0.0900 mol dm⁻³.

Answer [HCl] = 0.0900 mol dm⁻³

Section 6 of 6Exam Checklist & Tips

7.1.3 – Key Points for the Exam

  • End point The point in a titration at which the indicator changes colour.
  • Reflux Heating a mixture with a condenser so that vapour condenses and returns to the flask.
  • Distillation Separation of liquids by differences in boiling point.
  • Recrystallisation Purification of a solid using its different solubility in hot and cold solvent.
  • Titre The volume of solution delivered from the burette to reach the end point.

Common Mistakes

  • Placing a stopper in the top of a heated reflux condenser, which creates a dangerous pressure build-up.
  • Averaging titres that are not concordant; only titres within about 0.10 cm³ of each other should be averaged.
  • Adding too much hot solvent in recrystallisation, which leaves the product dissolved and reduces the yield.
  • Placing the thermometer too high in a distillation set-up, which gives a boiling point reading that is too low.

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