DSE Chemistry: acids, bases and titration (tutor lesson plan)

2026-11-03

Tutor lesson-prep guides · Secondary Chemistry

Students can recite pH equals minus log of hydrogen ion concentration but freeze when asked why a weak acid of the same concentration has a higher pH. This lesson starts from particles and ends at the endpoint.

Objectives — Students can: (1) explain strong versus weak acids by degree of ionisation and use it to account for the pH difference at equal concentration; (2) complete a titration calculation with n = cV and a mole ratio; (3) choose an indicator for strong acid-strong base, weak acid-strong base and strong acid-weak base titrations, giving the equivalence pH.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Moles and concentration	The units in n = cV	mol, mol dm-3, dm3
Ionisation	What HCl becomes in water	H+ and Cl-
Logarithms	The log of 10 to the power -3	-3

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Strong versus weak	Compare ionisation and conductivity at equal concentration	Say “partly ionised”
8-18 min	pH and dilution	0.100 M HCl has pH 1; tenfold dilution adds one	Both worked by the student
18-30 min	Mole ratio in titration	25 cm3 of base against 20.0 cm3 of acid, step by step	n = 2.50 x 10-3 mol
30-40 min	Choose the indicator	One example for each of the three combinations, with reasons	All three correct
40-45 min	Exit check	Mark on the spot, correct on the spot	Three out of three

03 Teaching script (read it out)

Scene 1: Strong and weak differ in ionisation, not in concentration

Hydrochloric acid is strong and ionises almost completely, so a 0.100 M solution has a hydrogen ion concentration of 0.100 M and a pH of 1. Ethanoic acid is weak: at the same concentration only about one per cent ionises, so the hydrogen ion concentration is far lower and the pH is about 3.
Three observations confirm it at equal concentration: the weak acid has a higher pH, conducts less well, and reacts more slowly with carbonate. All three point to the same cause, fewer free hydrogen ions, rather than the empty phrase “less acidic”.
Open with the question of which has the lower pH at equal concentration and volume, and refuse an answer that stops at “stronger acid” without naming ionisation.

Ask the student: why does ethanoic acid of the same concentration have a higher pH than hydrochloric acid?

Scene 2: pH and dilution: tenfold dilution adds one to pH

pH is minus the log of the hydrogen ion concentration, so 0.100 M gives pH 1 and 0.010 M gives pH 2. Dividing the concentration by ten adds one to the pH, because the log of ten is one, not because the pH is multiplied by ten.
The rule holds only for a strong acid well above the self-ionisation of water. Near 10-7 M, water dominates and the pH approaches 7 rather than climbing indefinitely, which is why a dilute acid never reaches pH 9. Say this explicitly or students will compute it.
Drill the conversion both ways: a pH difference of one is a tenfold change in concentration, and a difference of two is a hundredfold.

Ask the student: when pH falls from 2 to 1, by what factor does the hydrogen ion concentration change?

Scene 3: Titration: moles first, endpoint second

Take 25.0 cm3 of 0.100 M sodium hydroxide against hydrochloric acid. The moles of base are n = cV = 0.100 times 0.025 = 2.50e-03 mol. The acid and the base react one to one, so the acid has the same number of moles. At 0.125 M the volume needed is V = n / c = 2.50e-03 / 0.125 = 0.020 dm3, which is 20.0 cm3.
Three fixed steps: write the equation, read the mole ratio, convert with n = cV. Skip any of them and students multiply 25.0 by 20.0 with no idea why.
The indicator follows the equivalence pH: strong acid with strong base gives pH 7, so phenolphthalein or methyl orange both work; weak acid with strong base gives a pH above 7, so use phenolphthalein; strong acid with weak base gives a pH below 7, so use methyl orange.

Ask the student: why is the equivalence pH not 7 when a weak acid is titrated with a strong base?

04 Question types and worked steps

Type	Example	Steps	Answer
pH calculation	What is the pH of 0.010 M hydrochloric acid?	Write the hydrogen ion concentration, then take minus log	pH 2
Dilution	Dilute 0.100 M hydrochloric acid a hundredfold; what is the new pH?	Divide concentration by 100, add 2 to pH	pH 3
Titration calculation	How much 0.125 M acid neutralises 25.0 cm3 of 0.100 M base?	Find base moles, apply the one to one ratio, divide by acid concentration	20.0 cm3
Indicator choice	Which indicator for ethanoic acid with sodium hydroxide?	Decide the equivalence pH is above 7, then match the range	Phenolphthalein

Sanity check (teach this): Check the numbers: 0.100 M times 0.025 dm3 is 2.50e-03 mol; 2.50e-03 mol divided by 0.125 M is 0.020 dm3 or 20.0 cm3; 0.100 M strong acid has pH 1 and tenfold dilution gives pH 2. The three figures agree, so have the student key them into a calculator during the lesson.

05 Independent practice (3 questions)

Question	Answer	What to watch
How much 0.100 M hydrochloric acid neutralises 20.0 cm3 of 0.050 M sodium hydroxide?	10.0 cm3, since n = 1.00 x 10-3 mol and the ratio is one to one	Whether base moles are found before dividing by acid concentration
What is the hydrogen ion concentration at pH 3?	1 x 10-3 M	Whether ten to the power of minus pH is used rather than a multiplication
Where is the equivalence pH for ammonia with hydrochloric acid, and which indicator?	Below 7, so methyl orange	Whether the weak base leads to an acidic equivalence point

06 Common mistakes and what to say

Mistake	Student writes	Say this
pH treated as proportional to concentration	“Dilute tenfold, so divide the pH by ten”	“pH is a logarithm: concentration divided by ten means pH plus one.”
Mole ratio assumed to be one to one	Treating sulfuric acid and sodium hydroxide as one to one	“Write the equation first; sulfuric acid needs two hydroxide ions.”
Indicator chosen by colour alone	“Phenolphthalein turns pink, so that is the endpoint”	“The endpoint must match the equivalence pH, not just any colour change.”
Mixed units	Multiplying cm3 directly by mol dm-3	“Convert cm3 to dm3 by dividing by 1000 before multiplying.”

07 Homework (paper, section and question numbers)

DSE chemistry acids paper — Acids questions 1 to 4: write the equation, the mole ratio and the n = cV line in every answer, in 20 minutes.
Titration section of the same paper — Questions 5 to 7, including one indicator choice: give the equivalence pH as the reason.

08 Exit check (last 5 minutes)

Question	Answer	Decision
Hydrogen ion concentration at pH 2	1 x 10-2 M	Answering 2 M means pH is still treated as a concentration
Indicator for strong base with weak acid	Phenolphthalein	Answering methyl orange means the equivalence pH was not judged
25.0 cm3 converted to dm3	0.0250 dm3	Answering 0.25 means dividing by 100

09 Tutor FAQ

Must pH lie between 0 and 14?

No. Concentrated acids can go below 0 and concentrated bases above 14, though DSE questions usually stay inside the range.

Are the endpoint and the equivalence point the same?

Not exactly. The equivalence point is equal moles; the endpoint is the colour change, and the two differ slightly.

Why is the equivalence point of a weak acid titration less sharp?

The buffer region is longer, so the pH jump is gentler; the indicator is still chosen from the equivalence pH.

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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.