The shape of the lesson
By the end of the lesson, learners can
- The procedure: Run a titration in the simulator: fill the burette, add the indicator, and slow down to single drops as the colour starts to change.
- Results and calculations: Extended.
- Pipette a fixed volume of alkali; burette adds the acid.
- Methyl orange: red in acid, yellow in alkali. Thymolphthalein: colourless in acid, blue in alkali.
- Titre = final − initial reading; concordant = within 0.10 cm3.
- Extended: moles = concentration × volume in dm3, then use the mole ratio.
One picture every idea returns to
In an acid-base titration a volumetric pipette transfers a fixed volume of alkali (for example 25.0 cm3) into a conical flask, a few drops of indicator are added, and acid is run in from a burette until the indicator just changes colour: the end-point. The volume of acid added is the titre (final reading minus initial reading). A rough titre is followed by accurate titrations, adding acid drop by drop near the end-point, until concordant results (titres within 0.10 cm3 of each other) are obtained; their mean is used. Methyl orange is red in acid and yellow in alkali; thymolphthalein is colourless in acid and blue in alkali. Litmus is not used for titrations because its colour change is not sharp.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Drop by drop to the end-point.. A titration finds exactly how much acid neutralises a measured volume of alkali. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: The procedure | Run a titration in the simulator: fill the burette, add the indicator, and slow down to single drops as the colour starts to change. Project tab 6 of the lab (Stoichiometry lab, titration) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Results and calculations | Extended. | Numbered Heads Together |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Results and calculations, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Numbered Heads Together
Teams of four number off. Put up a question, give the team time to make sure every member can answer, then call a number: that student answers for the team, so nobody can leave the thinking to someone else.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The rough titre overshoots the end-point, so leave it out of the mean. | Titre = final reading minus initial reading, not the final reading alone. Use the mole ratio from the equation: with H 2 SO 4, moles of NaOH are twice the moles of acid. And convert cm 3 to dm 3 by dividing by 1000. |
Acid-base titrations: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how to carry out a titration to find the volume of dilute hydrochloric acid that neutralises 25.0 cm3 of sodium hydroxide solution.
State the colour of methyl orange (a) in the sodium hydroxide at the start and (b) at the end-point when the acid is in slight excess.
The burette reading is 0.50 cm3 before a titration and 24.70 cm3 after. Calculate the titre.
A student obtains titres of 23.90 (rough), 23.80, 23.85 and 23.75 cm3. Identify the concordant results and calculate the mean titre.
(Extended) 25.0 cm3 of sodium hydroxide solution is neutralised by 22.40 cm3 of 0.150 mol/dm3 hydrochloric acid. Calculate the concentration of the sodium hydroxide. Give your answer to 3 significant figures.
(Extended) 25.0 cm3 of 0.200 mol/dm3 sodium hydroxide is neutralised by 20.0 cm3 of dilute sulfuric acid. 2NaOH + H2SO4 → Na2SO4 + 2H2O. Calculate the concentration of the sulfuric acid.
Answer key and marking notes
- Use a volumetric pipette to put 25.0 cm3 of sodium hydroxide into a conical flask and add a few drops of methyl orange (or thymolphthalein).
- Fill a burette with the acid and record the initial reading.
- Add acid while swirling, then drop by drop near the end-point, until the indicator just changes colour; record the final reading.
- Repeat until concordant titres are obtained and find their mean.
- (a) yellow.
- (b) red (orange at the exact end-point).
- Titre = final reading − initial reading
- = 24.70 − 0.50 = 24.20 cm3
- Concordant titres: 23.80, 23.85 and 23.75 cm3 (all within 0.10 cm3); the rough titre is ignored.
- Mean = (23.80 + 23.85 + 23.75) ÷ 3 = 23.80 cm3
- Moles HCl = 0.150 × 22.40 ÷ 1000 = 0.00336 mol
- NaOH + HCl → NaCl + H2O, so moles NaOH = 0.00336 mol
- Concentration = 0.00336 ÷ 0.0250 = 0.134 mol/dm3
- Moles NaOH = 0.200 × 25.0 ÷ 1000 = 0.00500 mol
- Moles H2SO4 = 0.00500 ÷ 2 = 0.00250 mol
- Concentration = 0.00250 ÷ 0.0200 = 0.125 mol/dm3
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Methyl orange is added to sodium hydroxide solution, and hydrochloric acid is added from a burette. What is the colour change at the end-point?
- Ayellow to orange-red
- Bred to yellow
- Ccolourless to blue
- Dblue to colourless
What colour is thymolphthalein in an alkaline solution?
- Ablue
- Bcolourless
- Cyellow
- Dred
The burette reads 2.35 cm3 at the start of a titration and 26.15 cm3 at the end-point. What is the titre?
- A23.80 cm3
- B26.15 cm3
- C28.50 cm3
- D24.20 cm3
A student records these titres: rough 25.40, then 24.90, 24.95, 25.30 and 24.85 cm3. Which titres are concordant?
- A24.90, 24.95 and 24.85 cm3
- Ball five titres
- C25.40 and 25.30 cm3
- D24.90, 24.95, 25.30 and 24.85 cm3
Why is acid added drop by drop as the end-point of a titration is approached?
- Aso the end-point is not overshot
- Bso the indicator does not react with the acid
- Cso the solution does not get too hot
- Dso the burette does not run dry
25.0 cm3 of sodium hydroxide solution is neutralised by 21.60 cm3 of 0.0500 mol/dm3 sulfuric acid. 2NaOH + H2SO4 → Na2SO4 + 2H2O. What is the concentration of the sodium hydroxide?
- A0.0864 mol/dm3
- B0.0432 mol/dm3
- C0.00216 mol/dm3
- D0.0250 mol/dm3
25.0 cm3 of barium hydroxide solution needs 18.75 cm3 of 0.100 mol/dm3 hydrochloric acid for neutralisation. Ba(OH)2 + 2HCl → BaCl2 + 2H2O. What is the concentration of the barium hydroxide?
- A0.0375 mol/dm3
- B0.0750 mol/dm3
- C0.150 mol/dm3
- D0.00188 mol/dm3
In a titration a student forgets to rinse the burette with the acid, so it still contains some water. What is the effect on the titre and on the calculated concentration of the alkali?
- Atitre too large; calculated concentration of alkali too high
- Btitre too small; calculated concentration of alkali too low
- Ctitre too large; calculated concentration of alkali too low
- Dno effect on either
Quiz key
Methyl orange is yellow in alkali and red in acid, so as acid is added the colour changes from yellow to orange at the end-point (red if acid is in excess). Red to yellow would be acid being titrated with alkali. Colourless to blue and blue to colourless are the colours of thymolphthalein, not methyl orange.
Thymolphthalein is blue in alkali and colourless in acid. Colourless is its colour in acid. Yellow is methyl orange in alkali, and red is methyl orange or litmus in acid.
Titre = final reading − initial reading = 26.15 − 2.35 = 23.80 cm3. 26.15 cm3 ignores the starting reading. 28.50 cm3 adds the readings instead of subtracting. 24.20 cm3 is a subtraction error.
24.90, 24.95 and 24.85 cm3 all lie within 0.10 cm3 of each other, so they are concordant and their mean (24.90 cm3) is used. The rough titre overshoots and is left out, so all five cannot be used. 25.40 and 25.30 include the rough titre. 25.30 is 0.45 cm3 away from the others, so it is not concordant.
Near the end-point a single drop changes the colour, so adding drop by drop means the exact volume needed is not exceeded. The indicator is meant to react to the change in pH; it does not need protecting. The heat released in a titration is small. The burette is refilled between titrations, so running dry is not the reason.
Moles of H2SO4 = 0.0500 × 21.60 ÷ 1000 = 0.00108 mol. Moles of NaOH = 2 × 0.00108 = 0.00216 mol. Concentration = 0.00216 ÷ 0.0250 = 0.0864 mol/dm3. 0.0432 forgets the 2 : 1 ratio. 0.00216 is the number of moles, not the concentration. 0.0250 is the volume of alkali in dm3, not a concentration.
Moles of HCl = 0.100 × 18.75 ÷ 1000 = 0.001875 mol. Moles of Ba(OH)2 = 0.001875 ÷ 2 = 0.0009375 mol. Concentration = 0.0009375 ÷ 0.0250 = 0.0375 mol/dm3. 0.0750 ignores the ratio. 0.150 multiplies by 2 instead of dividing. 0.00188 is the moles of HCl, not a concentration.
The water dilutes the acid, so more of it is needed to neutralise the alkali: the titre is too large. The calculation assumes the original acid concentration, so it gives too many moles of acid and therefore too high a concentration of alkali. A smaller titre would need a more concentrated acid. A larger titre cannot give a lower calculated concentration. Dilution always has an effect.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.