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4.2.9Chemical Changes

Required Practical- Strong Acid and Strong Alkali Titration

5 min read 5 parts Lesson 14 of 21 Unit quiz

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Section 1 of 5Equipment and Set-Up

Accurate apparatus is needed so that the volumes measured in the titration are reliable.

Pipette
  • Pipette A pipette measures a fixed volume, such as 25.0 cm³ of alkali, more precisely than a measuring cylinder can.
Burette
  • Burette A burette delivers the acid drop by drop, and its readings are taken to the nearest 0.05 cm³.
Indicator
  • Indicator A few drops of indicator, such as phenolphthalein, change colour at the end point of the reaction.
Conical flask
  • Conical flask The flask holds the alkali and indicator, and it is swirled so the acid mixes and the colour change is clear.

Section 2 of 5Method

Follow these steps in order to obtain a set of titres.

1
Use a pipette to transfer 25.0 cm³ of alkali into a clean conical flask, then add a few drops of indicator.
2
Fill the burette with acid, remove any air bubbles from the tip, and record the starting reading.
3
Add acid steadily while swirling the flask, then slow down to drop by drop as the colour change approaches.
4
Stop at the first permanent colour change and record the final reading; the difference is the titre.
5
Repeat until you have concordant results, meaning titres close together, typically within 0.10 cm³ of each other, then calculate the mean.

Section 3 of 5Safety and Accuracy

Good technique affects both the safety of the experiment and the quality of the data.

Safety and Accuracy
Safety
  • Safety Wear eye protection, because strong acids and strong alkalis can cause serious burns and eye damage.
Reading
  • Reading Read the burette at eye level and use the bottom of the meniscus, not the top.
Reliability
  • Reliability Repeat titrations to check results, and leave out anomalous titres when calculating the mean.
End point
  • End point Add the acid drop by drop near the end point so the colour change is not overshot.

Section 4 of 5Worked Example

Question

  • 25.0 cm³ of sodium hydroxide solution of concentration 0.100 mol/dm³ is neutralised by 24.50 cm³ of hydrochloric acid.
  • Calculate the concentration of the acid.
  • NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

  1. Moles of NaOH = concentration × volume in dm³ = 0.100 × (25.0 ÷ 1000) = 0.00250 mol.
  2. The equation shows a 1:1 molar ratio, so moles of HCl = 0.00250 mol.
  3. Volume of acid in dm³ = 24.50 ÷ 1000 = 0.02450 dm³.
  4. Concentration of HCl = moles ÷ volume = 0.00250 ÷ 0.02450 = 0.102 mol/dm³ (3 significant figures).

Answer The hydrochloric acid has a concentration of 0.102 mol/dm³.

Section 5 of 5Exam Checklist & Tips

4.2.9 – Key Points for the Exam

  • Titration A method of finding the volume of one solution that reacts exactly with a known volume of another.
  • End point The point at which the indicator changes colour, showing the reaction is just complete.
  • Concordant results Repeat titres that are close together, typically within 0.10 cm³ of each other.
  • Titre The volume of solution delivered from the burette in one titration.
  • Neutralisation A reaction between an acid and an alkali that produces a salt and water.

Common Mistakes

  • Reading the burette from the top of the meniscus instead of the bottom at eye level.
  • Rinsing the conical flask with alkali before use;
    • it should be rinsed with distilled water only, otherwise extra alkali is added.
  • Averaging all titres including the rough one; the rough titre is only a guide, so use concordant titres.
  • Converting cm³ to dm³ by multiplying by 1000 instead of dividing by 1000.
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