DSE Chemistry | Acids and Alkalis Tutor Prep Guide | pH, Titration and Neutralisation

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students can say an acid gives H+ ions but treat pH as a formula to memorise. This lesson puts pH back where it belongs: pH is just concentration written the other way round, and a tenfold change in concentration moves pH by one.

Objectives — Students can: (1) calculate the pH of a strong acid from its concentration and explain why a weak acid of the same concentration has a higher pH; (2) use n = cV to find the volume needed to neutralise; (3) use q = mc dT to find the enthalpy of neutralisation and explain the gap from the literature value.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Mole and concentration	Which volume unit does n = cV need	dm3, so cm3 divided by 1000
Logarithms	What is log 10	1
Energy units	How many joules in 1 kJ	1000 J

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Recall n = cV	three ways to state concentration	student computes 25 cm3 x 0.1 M
8-20 min	pH as a mirror of concentration	pH of 0.010 M HCl	student gets pH 2.00
20-32 min	Why weak acids sit higher	0.10 M ethanoic acid	student gets pH 2.87
32-44 min	Titration mole ratio	how many cm3 of 0.125 M HCl	student gets 20.0 cm3
44-54 min	Enthalpy of neutralisation	q = mc dT then divide by moles	student gets 58.8 kJ/mol
54-60 min	Exit check	three questions collected and marked	done independently

03 Teaching script (read it out)

Scene 1: pH is not new, it is the same concentration in another form

0.010 M HCl ionises completely, so [H+] = 0.010 M and pH = -log 0.010 = 2.00. Dilute tenfold to 0.0010 M and pH becomes 3.00. Write both lines side by side so students see concentration divided by ten, pH plus one.

Board focus: pH is not new, it is the same concentration in another form

Scene 2: Weak acid: the pH is higher because most molecules never ionise

0.10 M ethanoic acid with Ka = 1.8 x 10^-5 gives [H+] = sqrt(Ka c) = 1.34e-03 M, pH = 2.87. Hydrochloric acid at the same 0.10 M gives pH 1.00; the 1.87 gap is simply the fraction of molecules that released a proton.

Board focus: Weak acid: the pH is higher because most molecules never ionise

Scene 3: Neutralisation enthalpy: heat first, then moles

Mix 25.0 cm3 of 0.100 M NaOH with 25.0 cm3 of 0.125 M HCl: total mass 50 g, temperature rise 0.7 K. q = mc dT = 147 J, n = 0.0025 mol, so enthalpy = 58.8 kJ/mol. The literature value is about 57.3; the difference is heat loss and thermometer error.

Board focus: Neutralisation enthalpy: heat first, then moles

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	pH of a strong acid	pH = -log c	2.00
Calculation	pH of a weak acid	sqrt(Ka c) first, then log	2.87
Titration	1:1 neutralisation	c1V1 = c2V2	20.0 cm3
Energy	enthalpy of neutralisation	q = mc dT divided by n	58.8 kJ/mol

Sanity check (teach this): Fix the units first: always convert cm3 to dm3 before multiplying by concentration. The square-root shortcut for a weak acid only holds when Ka is small; otherwise solve the quadratic. A calorimetry answer is always high if heat taken by the cup is ignored.

05 Independent practice (3 questions)

Question	Answer	What to watch
pH of 0.010 M HCl	2.00	-log 0.010
pH of 0.10 M ethanoic acid, Ka = 1.8 x 10^-5	2.87	sqrt(1.8e-5 x 0.10)
Volume of 0.125 M HCl to neutralise 25.0 cm3 of 0.100 M NaOH	20.0 cm3	n = cV then divide
Enthalpy when the mixture warms by 0.7 K	58.8 kJ/mol	q = mc dT divided by n

06 Common mistakes and what to say

Mistake	Student writes	Say this
Multiplying cm3 straight by concentration	25 x 0.100 = 2.5 mol	“25 is cm3 not dm3. Divide by 1000 first and you get 0.0025.”
Treating a weak acid as strong	pH of 0.10 M ethanoic acid = 1.00	“Only about one percent ionises, so the pH is 2.87, not 1.”
Forgetting to divide by moles	answering 147 kJ/mol	“That number is joules for the whole mixture. Divide by 0.0025 mol to get per mole.”
Wrong mole ratio in titration	treating H2SO4 as 1:1	“One H2SO4 gives two protons, so the volume halves.”

07 Homework (paper, section and question numbers)

DSE Chemistry practice paper (acids and titration) — Q1-Q4
DSE Chemistry practice paper (energy changes) — Q7-Q9

08 Exit check (last 5 minutes)

Question	Answer	Decision
pH of 0.010 M HCl	2.00	correct means ready for buffers
pH of 0.10 M ethanoic acid	2.87	wrong means reteach Ka next lesson
Volume of 0.125 M HCl for 25.0 cm3 of 0.100 M NaOH	20.0 cm3	wrong means redrill n = cV

09 Tutor FAQ

Do students need a calculator for pH?

Paper 1 does not allow one, so only powers of ten such as 0.10 and 0.010 appear; drill them mentally.

Why is neutralisation enthalpy not identical for every acid?

A strong acid and base are fully ionised, so the reaction is always H+ plus OH-. A weak acid must ionise first and absorbs part of the energy.

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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.