DSE Chemistry | Redox Titration Calculations: a tutor guide with electron counting and five steps

2026-11-05

Students who titrate acids confidently still forget the electron multiplier in redox, or report the answer for one aliquot as if it were the whole flask. This lesson locks three moves: count electrons for the ratio, line the numbers up in five steps, then scale the aliquot back up.

Objectives — Students can: (1) derive the mole ratio from the electron counts of two half-equations and explain why it is 5 to 1; (2) find an unknown concentration with moles, ratio and volume in that order; (3) finish a back titration, scale up to the whole flask and report a percentage composition.

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 in 20.0 cm3 of 0.0200 M solution?	4.00 x 10-4 mol
Half-equation balancing	How many electrons does MnO4- take in acid to become Mn2+?	Five, so five Fe2+ are needed
Significant figures	How many figures for 0.0200 M and 20.0 cm3?	Three

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Three slots	Write ratio, volume, scale-up on the board; ask which slot is most often missed	Name the slot they missed last time
5-14 min	Count electrons	Write two half-equations, count electrons, conclude 5 Fe2+ to 1 MnO4-	Balance them and circle the electrons
14-24 min	Five-step direct titration	Model moles of MnO4-, times five, divide by sample volume, times molar mass, units	Repeat the same data themselves
24-33 min	Back titration and aliquot	Make the 250.0 to 25.0 jump explicit and work back to the flask	Draw the flask, aliquot, scale-up arrows
33-40 min	One exam question	Full run with every intermediate line	Work alone and mark the missing step
40-45 min	Exit check	Collect and mark three questions	Work alone

03 Teaching script (read it out)

Scene 1: Count electrons first, and the ratio stops being a chant

Students learn 5 to 1 as a chant, then collapse when the question switches to Cr2O7 2-. Open with one move: write Fe2+ to Fe3+ plus an electron, and MnO4- plus 8H+ plus 5 electrons to Mn2+ plus 4H2O, then ask who hands electrons to whom and how many. The answer is the ratio: five Fe2+ deliver five electrons, exactly enough for one MnO4-.
Put count electrons at the top of the board and repeat it for every new oxidising agent.

Students do: circle the electrons and write one sentence saying five Fe2+ trade with one MnO4-.

Scene 2: Five steps: moles, ratio, volume, mass, unit

Errors are order errors, not arithmetic: a missing factor of five, cm3 left unconverted, or grams where mol per dm3 belongs. Fix five steps: moles of MnO4-, multiply by the ratio, divide by sample volume, multiply by molar mass, write the unit.
Model it: 25 cm3 of sample needs 20 cm3 of 0.02 M KMnO4, so n is 4.00e-04 mol, iron is 2.00e-03 mol, the concentration is 0.0800 M and the mass is 0.1116 g. Every student keys the numbers in personally.

Students do: write the five steps down the left margin and tick each one.

Scene 3: Back titration: ask what fraction one aliquot is before scaling up

The classic error is reporting the aliquot answer as the whole flask. Draw three arrows first: 250.0 cm3 in the flask, 25.0 cm3 taken out, times ten back to the flask. Every student says out loud that the aliquot is one tenth.
Model it: 1.2 g of ore is made up to 250 cm3, 25 cm3 of it needs 18.4 cm3 of 0.02 M KMnO4. That is 3.68e-04 mol of permanganate, 1.84e-03 mol of iron in the aliquot, 1.840e-02 mol in the flask, 1.0267 g, and 85.6 percent by mass.

Students do: write times ten next to the answer and circle it before writing any percentage.

04 Question types and worked steps

Type	Example	Steps	Answer
Direct titration	25 cm3 of sample needs 20 cm3 of 0.0200 M KMnO4	Moles, times five, divide by volume	0.0800 M
Mass in the sample	Same data, how many grams of iron?	One more step: multiply by molar mass	0.1116 g
Back titration purity	1.2 g ore, 250.0 cm3, aliquot 25.0 cm3, 18.40 cm3 used	Aliquot, times ten, divide by sample mass	85.6 percent
Error analysis	The endpoint overshoots by 0.10 cm3; how high is the result?	Extra volume over the original volume	About 0.54 percent high

Sanity check (teach this): Sanity check: electrons must balance, so five Fe2+ pair with one MnO4- and a 1 to 1 ratio is wrong; permanganate is its own indicator and the endpoint is a persistent pale pink for 30 seconds, never deep purple; the unit is mol per dm3, so cm3 must be converted first; a purity of 85.6 percent is believable because ore carries impurities, and anything above 100 percent means a scale-up or weighing error; keep three significant figures when the data give three.

05 Independent practice (3 questions)

Question	Answer	What to watch
Direct titration	25 cm3 of sample needs 20 cm3 of 0.0200 M KMnO4; find the iron concentration	Check the factor of five and the cm3 conversion
Mass	Same data; find the mass of iron in 25 cm3	Check 55.8 rather than 56
Back titration	1.20 g ore, 250.0 cm3, aliquot 25.0 cm3, 18.40 cm3; find the percentage	Check times ten and the denominator

06 Common mistakes and what to say

Mistake	Student writes	Say this
Missing electron multiplier	n(Fe2+) = 4.00 x 10-4 mol	“How many electrons does MnO4- accept? Where did the five Fe2+ go?”
cm3 not converted	c = 4.00 x 10-4 divided by 20.0	“The denominator is dm3. Are you dividing by cm3 or dm3?”
No scale-up in back titration	Answer given as 0.103 g	“What fraction of the flask is this aliquot? Have you scaled it up?”
Endpoint called too early	Stops at the first hint of purple	“Pale pink for 30 seconds is the endpoint; deep purple means you have overshot.”

07 Homework (paper, section and question numbers)

Redo this chapter (Paper 1 Q7 style) — Two back titration questions with all five steps and question numbers written
Secondary chemistry index — Other chapters and their paper question types
All prep guides — The same ten-section structure in other subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Mole ratio	Five Fe2+ to one MnO4-, justified by electrons	A 1 to 1 answer means repeat act one
Five steps	Moles, ratio, volume, molar mass, unit	A missing ratio means repeat act two
Scale-up	Times ten before the percentage	No scale-up means repeat act three

09 Tutor FAQ

Should calculators be banned when arithmetic keeps failing?

No. Require every intermediate value on paper before keying, so errors show as copying.

Must students memorise the electron counts?

Not by heart, but they must balance the MnO4- and Cr2O7 2- half-equations in acid.

Is the flask volume always given?

Usually, but when the wording says made up to 250.0 cm3, students must recognise that as the whole flask.

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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.