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DSE Chemistry: Acid-Base Titration and pH — Tutor Lesson Plan
Students compute a 25.0 cubic centimetre titre but answer “why this indicator?” with “because the teacher said phenolphthalein”.
Objectives — The student can compute pH from hydrogen ion concentration, complete titration calculations including a diprotic acid, and justify an indicator from the pH at the equivalence point.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| n = cV | Volume in cubic decimetres, so divide by 1000 | 25.0 cubic cm of 0.100 M is 2.50 mmol |
| Neutralisation | H+ plus OH- gives water | Sulfuric acid supplies two H+ per molecule |
| Logarithms | Each pH unit is a factor of ten | pH 2 is ten times more acidic than pH 3 |
02 Lesson flow (60 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-8 min | Prerequisite check | Three n = cV drills, one given in cubic centimetres | Converts everything first |
| 8-20 min | pH and acid strength | 0.0100 M HCl gives pH 2.00; a weak acid only partly ionises | Says the word ionisation |
| 20-34 min | Titration calculation | 25.0 cubic cm of 0.100 M NaOH needs 25 cubic cm HCl; with sulfuric acid still 25 but only 1.25 mmol | Writes the 1:2 ratio |
| 34-46 min | Curves and indicators | Strong-strong equivalence at 7; weak acid with strong base 8 to 9; strong acid with weak base 4 to 5 | Matches three indicators |
| 46-54 min | Data handling | Titres 24.80, 24.85, 25.60: discard the last, mean 24.825 | Justifies the rejection |
| 54-60 min | Exit check | Three questions marked at once | Works alone |
03 Teaching script (read it out)
Scene 1: pH is a concentration measure, not a label
pH = minus log of the hydrogen ion concentration. 0.0100 M HCl ionises fully, so the concentration is 0.0100 M and pH is 2.00.
A 0.0100 M acetic acid solution ionises only about one percent, giving roughly 10 to the minus 4 M and a pH near 4. Same bottle concentration, two pH units apart, and the whole difference is the degree of ionisation.
The sentence to fix in their heads: pH reports free hydrogen ions, not the concentration on the label. So a tenfold dilution of a strong acid raises pH by 1, while the same dilution of a weak acid raises it only about 0.5.
A 0.0100 M acetic acid solution ionises only about one percent, giving roughly 10 to the minus 4 M and a pH near 4. Same bottle concentration, two pH units apart, and the whole difference is the degree of ionisation.
The sentence to fix in their heads: pH reports free hydrogen ions, not the concentration on the label. So a tenfold dilution of a strong acid raises pH by 1, while the same dilution of a weak acid raises it only about 0.5.
Ask for the sentence: pH comes from free hydrogen ions, not from the labelled concentration.
Scene 2: Titration: ratio first, arithmetic second
Fixed routine: write the equation, compute moles on the known side with unit conversion, apply the coefficient ratio, divide by the concentration to get the volume.
25.0 cubic cm of 0.100 M NaOH holds 2.50 mmol of hydroxide, so 0.100 M HCl needs 25 cubic cm.
Swap to 0.0500 M sulfuric acid and the acid moles are 1.25 mmol, but each molecule delivers two hydrogen ions, so 2.50 mmol of hydroxide is required and the volume is still 25 cubic cm of 0.100 M NaOH. Half the concentration, same volume, because one molecule hits twice. This is the single most dropped mark in DSE titration.
25.0 cubic cm of 0.100 M NaOH holds 2.50 mmol of hydroxide, so 0.100 M HCl needs 25 cubic cm.
Swap to 0.0500 M sulfuric acid and the acid moles are 1.25 mmol, but each molecule delivers two hydrogen ions, so 2.50 mmol of hydroxide is required and the volume is still 25 cubic cm of 0.100 M NaOH. Half the concentration, same volume, because one molecule hits twice. This is the single most dropped mark in DSE titration.
Ask the student: the mole ratio between sulfuric acid and sodium hydroxide, and why it is 1 to 2.
Scene 3: The curve chooses the indicator
Strong acid with strong base: equivalence at pH 7, but the curve is almost vertical from pH 3 to 10, so either phenolphthalein (8.2 to 10.0) or methyl orange (3.1 to 4.4) crosses it cleanly.
Weak acid with strong base, for example acetic acid and sodium hydroxide: equivalence is basic, near pH 8 to 9, so phenolphthalein is required. Methyl orange changes before the equivalence point and the titre comes out low.
Strong acid with weak base, for example hydrochloric acid and ammonia: equivalence is acidic, near pH 5, so methyl orange is the choice.
On data: the three titres 24.80, 24.85 and 25.60 cubic cm differ by more than 0.7 in the last reading, which means the endpoint was overshot and that value is discarded. The mean of the first two is 24.825 cubic cm, and they may only be averaged because they agree within 0.05.
Weak acid with strong base, for example acetic acid and sodium hydroxide: equivalence is basic, near pH 8 to 9, so phenolphthalein is required. Methyl orange changes before the equivalence point and the titre comes out low.
Strong acid with weak base, for example hydrochloric acid and ammonia: equivalence is acidic, near pH 5, so methyl orange is the choice.
On data: the three titres 24.80, 24.85 and 25.60 cubic cm differ by more than 0.7 in the last reading, which means the endpoint was overshot and that value is discarded. The mean of the first two is 24.825 cubic cm, and they may only be averaged because they agree within 0.05.
Remind them: a rejected value needs a stated reason, not “it looked different”.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| pH calculation | pH of 0.0100 M HCl | Full ionisation, so concentration is the hydrogen ion concentration | 2.00 |
| Titre volume | 25.0 cubic cm of 0.100 M NaOH against 0.100 M HCl | Moles, then the 1:1 ratio, then divide by concentration | 25 cubic cm |
| Diprotic acid | 0.0500 M sulfuric acid, 25.0 cubic cm, against NaOH | Double the acid moles, then divide | 25 cubic cm |
| Indicator choice | Weak acid with strong base | Locate the equivalence pH first | Phenolphthalein, equivalence near 8 to 9 |
Sanity check (teach this): Sanity-check titres by order of magnitude: 0.1 M solutions against each other always land in the tens of cubic centimetres, so 0.25 or 2,500 means a missed conversion or a missed coefficient. For a mixture, work out which side is in excess: 25.0 cubic cm of 0.100 M HCl plus 15.0 cubic cm of 0.100 M NaOH leaves 1.00 mmol of hydrogen ions in 40 cubic cm, a concentration of 0.025 M and a pH of 1.60. It is not 7, and averaging the two starting pH values is meaningless.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| pH of 0.0010 M HCl | 3.00 | Do they go straight to minus log of the concentration |
| 25.0 cubic cm of 0.100 M NaOH against 0.0500 M sulfuric acid: volume? | 25 cubic cm | Most answer 50, having missed the 1:2 ratio |
| How many times more acidic is pH 2 than pH 5? | 1000 times | Watch for anyone treating a difference of 3 as a factor of 3 |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Volumes left in cubic centimetres | Computes 0.100 times 25.0 as 2.5 mol | “Divide the volume by a thousand before you multiply.” |
| Missing the diprotic coefficient | Treats sulfuric acid and hydroxide as 1 to 1 | “One sulfuric acid molecule releases two hydrogen ions, so it needs twice the base.” |
| Averaging pH values | Calls pH 1 plus pH 3 equal to pH 2 | “pH is logarithmic; go back to hydrogen ion concentration before you mix.” |
| Choosing indicators by habit | Uses phenolphthalein for everything | “Find the equivalence pH first, then pick the indicator whose range brackets it.” |
07 Homework (paper, section and question numbers)
- DSE Chemistry paper 1 — Section A multiple choice, first 3 items: pH and indicator choice
- Chemistry chapter index — Section B structured question 1: titration calculation with the mole ratio written out
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| pH of 0.0100 M HCl | 2.00 | Three correct means move on to electrochemistry |
| 0.0500 M sulfuric acid, 25.0 cubic cm, against 0.100 M NaOH: volume | 25 cubic cm | One wrong means reteach the mole ratio |
| Indicator for a weak acid with a strong base | Phenolphthalein | Two wrong means redraw the titration curves |
09 Tutor FAQ
Must pH formulas be memorised?
Yes: pH is minus log of hydrogen ion concentration, and the reverse is ten to the minus pH.
Should every titre be averaged?
Only values agreeing within 0.05 cubic cm; outlying values are discarded with a stated reason.
Can any indicator be used?
No. Its range must bracket the equivalence pH, otherwise titres are systematically low.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
