DSE Chemistry | Titration Curves and Indicator Choice Tutor Prep Guide | 45-minute plan, equivalence pH, four common errors

2026-11-05

Tutor lesson-prep guides · Secondary Chemistry

What this lesson fixes: students write phenolphthalein on every titration question without checking whether the acid is strong or weak, so the equivalence pH, the indicator and the endpoint all go wrong together.

Objectives — By the end the student can (1) decide within 40 seconds whether the equivalence pH is above, equal to or below seven from the strong-weak combination; (2) sketch the strong acid with strong base, weak acid with strong base and strong acid with weak base curves and mark the vertical section; (3) choose a usable indicator from its colour-change range and complete the titration calculation using the mole ratio.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Molar concentration	How many moles are in 25.0 cubic centimetres of 0.100 M sodium hydroxide	Not reaching 2.50 times 10 to the minus 3 mol means moles = M times V needs revision
Strong versus weak	Sort HCl, ethanoic acid, sodium hydroxide and ammonia into strong and weak	Calling ethanoic acid strong means this needs repair first
pH definition	What is the pH of 0.100 M hydrochloric acid	Anything other than 1 means pH = minus log of hydrogen ion concentration is missing

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
5 min	Opening diagnostic	Four substances on the board to be sorted into strong acid, weak acid, strong base, weak base	Answer orally; every item must be correct before moving on
8 min	Three curve families	Draw strong-strong, weak-strong and strong-weak side by side, marking only the start, the equivalence point and the vertical section	Copy each and write the equivalence pH
10 min	Indicator choice	Lay the methyl orange and phenolphthalein ranges over the three curves; the vertical section must cover the range	Write the usable indicator next to each curve
10 min	Titration calculation	Model 25.0 cubic centimetres of 0.100 M sodium hydroxide against 0.100 M hydrochloric acid	Follow along; mole ratio before volume
8 min	One independent question	Weak acid with strong base: sketch, fix the equivalence pH, choose the indicator	Tutor observes only
4 min	Exit check	Three quick questions	Retach anything wrong immediately

03 Teaching script (read it out)

Scene 1: Act one: what actually sets the equivalence pH

Students assume the equivalence point must be pH 7, which is the most expensive misconception in this topic. The equivalence pH is set by the salt produced. A strong acid with a strong base gives a salt that does not hydrolyse, so the pH is seven. A weak acid with a strong base gives a basic salt that hydrolyses, so the pH is above seven, around 8.7 for sodium ethanoate. A strong acid with a weak base gives an acidic salt, around 5.3 for ammonium chloride. The first question is therefore always what the other partner is, never what colour the indicator turns.

Ask: what decides the equivalence pH?

Scene 2: Act two: the vertical section is what selects the indicator

The point of a titration curve is not how pretty it looks but the near-vertical jump in pH around the equivalence point. Strong acid with strong base jumps from about pH 3 to about 11, so both methyl orange (3.1 to 4.4) and phenolphthalein (8.3 to 10.0) sit inside it and either works. Weak acid with strong base jumps only from about pH 7 to 10, so methyl orange finishes changing far too early and phenolphthalein is compulsory. Strong acid with weak base jumps from 3 to 7, so phenolphthalein changes far too late and methyl orange is compulsory. A slogan will not survive the exam; drawing the range as a horizontal band on the curve will.

Ask the student to draw the methyl orange band and say whether it covers pH 8.7.

Scene 3: Act three: the calculation is only a mole ratio

Take 25.0 cubic centimetres of 0.100 M sodium hydroxide titrated with 0.100 M hydrochloric acid. Moles of base equal 0.100 times 25.0 over 1000, which is 2.50 times 10 to the minus 3 mol. The reaction is one to one, so the acid requires the same number of moles, and the volume is that divided by 0.100, giving 25.0 cubic centimetres. Change the acid to 0.050 M sulphuric acid and the volume is still 25.0 cubic centimetres, but the moles of acid are halved because one mole of the diprotic acid neutralises two moles of base. Students should explain that themselves; if they can, the topic is secure.

Ask why the sulphuric acid volume stays the same while the moles halve.

04 Question types and worked steps

Type	Example	Steps	Answer
Strong acid, strong base	25.0 cubic centimetres of 0.100 M sodium hydroxide against 0.100 M hydrochloric acid	Equivalence pH 7, vertical section 3 to 11, either indicator	Both are acceptable and the answer must say why
Weak acid, strong base	25.0 cubic centimetres of 0.100 M ethanoic acid against 0.100 M sodium hydroxide	Equivalence pH about 8.7, vertical section 7 to 10, phenolphthalein only	Methyl orange changes far too early and the answer must say so
Strong acid, weak base	25.0 cubic centimetres of 0.100 M hydrochloric acid against 0.100 M ammonia	Equivalence pH about 5.3, vertical section 3 to 7, methyl orange only	Phenolphthalein only starts at 8.3, well past the endpoint
Titration calculation	How much 0.050 M sulphuric acid neutralises 25.0 cubic centimetres of 0.100 M sodium hydroxide	Write the one to two mole ratio before any numbers	2.50 times 10 to the minus 3 mol of base, 1.25 times 10 to the minus 3 mol of acid, 25.0 cubic centimetres

Sanity check (teach this): The moles must reconcile across the whole question: 0.100 M in 25.0 cubic centimetres is 2.50 times 10 to the minus 3 mol and that number reappears later. The volume check is simple: at the equivalence point the solution has gone from 25.0 to 50.0 cubic centimetres, so the concentration has halved and the original 0.100 M cannot be reused when computing pH. For indicators, write the ranges as two bands, 3.1 to 4.4 and 8.3 to 10.0, and lay them against an equivalence pH of 5.3 or 8.7; whether the band covers the vertical section is visible at a glance.

05 Independent practice (3 questions)

Question	Answer	What to watch
20.0 cubic centimetres of 0.100 M sodium hydroxide against 0.100 M hydrochloric acid: equivalence volume and usable indicator	Moles first, then the one to one ratio, then the vertical section	20.0 cubic centimetres, either indicator
15.0 cubic centimetres of 0.200 M ethanoic acid against 0.150 M sodium hydroxide: equivalence volume	Moles = 0.200 times 0.0150, then divide by 0.150	3.00 times 10 to the minus 3 mol, 20.0 cubic centimetres
Explain why phenolphthalein fails for a strong acid with a weak base	Start from the equivalence pH, then the vertical section, then the range	All three layers required

06 Common mistakes and what to say

Mistake	Student writes	Say this
Equivalence pH always written as 7	Neutralisation means pH 7, so use phenolphthalein	Neutralisation means equal moles of acid and base, not equal pH. Tell me which salt is formed, then tell me whether the pH is above or below seven.
Methyl orange chosen for a weak acid with a strong base	Ethanoic acid with sodium hydroxide, indicator given as methyl orange	Methyl orange starts changing at 3.1 and your equivalence point is 8.7. Does your band cover the vertical section?
pH computed without allowing for dilution	The original 0.100 M reused at the equivalence point	At the equivalence point the total volume has gone from 25 to 50, so the concentration has halved. Did your line use the new volume?
Diprotic acid treated as one to one	0.050 M sulphuric acid counted one to one against sodium hydroxide	Sulphuric acid has two ionisable hydrogens, so one mole of acid needs two moles of base. Write the ratio down first.

07 Homework (paper, section and question numbers)

Titration technique and calculation consolidation on the same subject page — Section 06, questions 1 to 3
Secondary Chemistry prep index: preview the next organic chapter — Pick one organic chapter from the index and read the objectives section

08 Exit check (last 5 minutes)

Question	Answer	Decision
Weak acid titrated with strong base: is the equivalence pH above or below seven	Above seven, because the salt formed hydrolyses to give a basic solution	The reason is required, not just the direction
Why both indicators work for a strong acid with a strong base	The vertical section runs from about pH 3 to 11, so both ranges sit inside it	The vertical section must be mentioned
How many moles are in 25.0 cubic centimetres of 0.100 M sodium hydroxide	2.50 times 10 to the minus 3 mol	Value and unit both correct

09 Tutor FAQ

The student says memorising the colours is enough

Colours only answer colour questions. Titration questions ask whether an indicator can be used, which is decided by the equivalence pH and the vertical section, so the training is drawing the band on the curve.

Must the exact equivalence pH be memorised

Not to a decimal place, but the direction must be: seven for strong with strong, above seven for weak acid with strong base, below seven for strong acid with weak base, with rough windows of 7 to 10 and 3 to 7.

Is a school titration practical the same as the exam

Practicals usually involve standardising a solution, while exam questions reason back from data. The 45 minutes here trains the concepts; bench technique is separate.

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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.