Titrations and Strong and Weak Acids [chemistry only; HT for weak acids]

SPEC 4.4.2.5 / 4.4.2.6 • PAPER 1 • RP2

Titrations & Strong vs Weak Acids

RP2 — the most precision-tested practical at GCSE. Every apparatus choice has a marked reason. Every titration calculation depends on cm³ → dm³. And on Higher Tier, the strong/weak/concentrated distinction is the trap that catches the most marks every year.

What Is a Titration?

Titration is the most-examined practical technique on Paper 1. It appears in some form in every recent series and is the highest-stakes RP for chemistry-only candidates.

Titration

A technique for finding the volumes of acid and alkali that exactly react, using an indicator to detect the endpoint. On Higher Tier this allows the concentration of one solution to be calculated from the known concentration of the other.

RP2 Spec Limit

Spec 4.4.2.5 restricts RP2 to strong acids and strong alkalis only: sulfuric, hydrochloric and nitric acids only. (Weak acids would not give a sharp endpoint.) This is chemistry-only content — Combined Science students do not sit it.

Two Lessons in One

This lesson does two distinct jobs: (1) the practical method with apparatus and concordance — Foundation + Higher; (2) the strong/weak acid distinction with the strong-vs-concentrated trap — Higher only.

RP2 Apparatus — Every Choice Earns a Mark

Each apparatus name is a separate marking point. The mark scheme rejects the wrong vessel, wrong measuring device, or wrong indicator outright — even if the procedure is otherwise correct.

Volumetric pipette

For the alkali (typically 25.0 cm³). Delivers an accurate, reproducible fixed volume. NOT a measuring cylinder — not precise enough.

Conical flask

Holds the alkali during titration. NOT a beaker — sloping sides mean the contents can be swirled without splashing out.

Burette

For the acid of known concentration. Readable to 0.05 cm³ — the source of the precise volume measurement.

Indicator (single change)

Methyl orange or phenolphthalein — sharp colour change at endpoint. NOT universal indicator (continuous colour change = no sharp endpoint).

White tile

Placed under the conical flask so the colour change is easy to see against a white background.

Pipette filler

Used with the volumetric pipette — mouth-pipetting alkali is unsafe.

RP2 Method — Seven Steps

1

Pipette the alkali. Use a volumetric pipette (with filler) to transfer exactly 25.0 cm³ of alkali into a conical flask.

2

Add indicator. A few drops of phenolphthalein or methyl orange to the alkali in the flask.

3

Fill the burette. With the acid of known concentration. Record the initial reading to two decimal places (read from the bottom of the meniscus at eye level).

4

Add acid slowly from the burette, swirling the flask continuously. Place the conical flask on a white tile.

5

Slow to drop-by-drop near the endpoint. Stop at the first permanent colour change. Record the final burette reading.

6

Calculate the titre (final reading − initial reading).

7

Repeat until you obtain concordant titres (within 0.10 cm³ of each other). Calculate the mean of the concordant titres only .

What's NOT a Method "Improvement"

Examiner report (P1 Nov 2021 Q09.3): "Many students included reading the bottom of the meniscus at eye level; this is just normal technique rather than an improvement." Standard procedure ≠ improvement. Also: "Many suggested repeating the titration, but this is pointless unless a mean of the accurate results is then taken" — the repeat must be paired with "and calculate the mean of concordant results".

Indicators — Sharp Change Required

IndicatorIn alkaliIn acid (endpoint)

Phenolphthaleinpinkcolourless

Methyl orangeyellowred

Litmusbluered

Universal indicatorNOT suitable — continuous colour change gives no sharp endpoint

Mark-Scheme Language (P1 Jun 2023 Q05.3)

Phenolphthalein endpoint is "colourless" — NOT "clear". The examiner explicitly distinguishes these: "clear" describes transparency (any colour can be clear); "colourless" specifies the absence of colour. Mark scheme awards "colourless" only.

Direction Matters

In RP2 you add acid to alkali. So the indicator changes FROM its alkali colour TO its acid colour: phenolphthalein from pink → colourless; methyl orange from yellow → red. Reversing the direction loses the mark.

Concordant Titres — Within 0.10 cm³

Concordant Results

Repeat titration values that agree to within 0.10 cm³ of each other. Only the concordant titres are used to calculate the mean — anomalies are discarded.

The Rough Titration

The first titration is usually the rough titre — done quickly to find roughly where the endpoint is. It is almost always discarded. Repeat titrations 2 and 3 (slowing down near the endpoint) should agree within 0.10 cm³.

Examiner Alert (P1 Jun 2022 Q08.2)

Given titres 23.90, 23.45, 23.55, 23.55, 23.45 cm³, students were asked why the student used 23.50 cm³ in the calculation. Mark scheme: "23.90 cm³ is anomalous / not concordant / a rough value"; "the mean is taken of the concordant values (within 0.10 cm³)". Mean of all 5 = 23.58 — incorrect. Mean of titres 2–5 = 23.50 — correct.

Examiner Alert — RP2 Apparatus Errors

"Many students gave three or four correct steps in the procedure. However, many omitted to say that the alkali should be measured with a (volumetric) pipette, which is an essential step, so did not gain full marks. Most students remembered to add an indicator, although some suggested using universal indicator, which is not suitable for a titration. Carrying out the titration in a beaker did not gain credit because of the risk of spillage while swirling. Those who suggested taking the initial burette reading, or filling to a particular reading, often expressed themselves poorly; a burette does not give readings of 'amount.'" — AQA Examiner Report, P1 Jun 2024 Q09.3

✗ Wrong

"Measuring cylinder for the alkali"Mark scheme rejects — not precise enough.

✓ Right

"Volumetric pipette for the alkali"Spec 4.4.2.5 RPA2 — required first step.

✗ Wrong

"Beaker"Spillage risk when swirling.

✓ Right

"Conical flask"Sloping sides contain the swirl.

✗ Wrong

"Universal indicator"Mark scheme: "do not accept add universal indicator."

✓ Right

"Phenolphthalein / methyl orange"Sharp single-pH change.

Titration Calculations HT Only

Once you have the mean concordant titre and the known concentration, the HT spec asks you to calculate the unknown concentration. There are three steps. Always the same three steps.

The Three Steps

Moles of the known. moles = concentration × volume (in dm³). Convert cm³ to dm³ by dividing by 1000.

Apply the mole ratio from the balanced equation. Read the coefficients carefully: 1:1, 1:2, 2:1 — they're not always 1:1.

Concentration of the unknown. concentration = moles ÷ volume (in dm³).

moles = c × v | c = moles ÷ v | v in dm³

Unit Conversion

Volumes in titration questions come in cm³; the formula uses dm³. Divide cm³ by 1000 to get dm³. Forgetting this gives an answer that is 1000× too small (or too large, depending on which step).

mol/dm³ ↔ g/dm³ (Spec 4.4.2.5)

The HT spec covers calculations in both mol/dm³ AND g/dm³. Convert with: concentration in g/dm³ = concentration in mol/dm³ × Mr.

Example: 0.100 mol/dm³ NaOH; Mr(NaOH) = 40. So in g/dm³ = 0.100 × 40 = 4.00 g/dm³. Reverse direction: divide g/dm³ by Mr to get mol/dm³.

Worked Example — 1:1 Ratio HT Only

Q: The student titrated a solution containing 0.0045 moles of sodium hydroxide with 0.15 mol/dm³ hydrochloric acid. The equation for the reaction is: NaOH + HCl → NaCl + H₂O. Calculate the volume of hydrochloric acid in cm³ needed in the titration.2 MARKS

Citation: P1 Jun 2024 Q09.4 — verbatim AQA stem.

1

Mole ratio: NaOH : HCl = 1 : 1. So moles HCl = moles NaOH = 0.0045 mol.

2

Volume in dm³: v = moles ÷ c = 0.0045 ÷ 0.15 = 0.030 dm³.

3

Convert to cm³: 0.030 × 1000 = 30 cm³.

⚠️

Examiner caution: ER reports many students "multiplied moles and concentration together" — that gives the wrong unit. Volume = moles ÷ concentration. Always check units of the answer match the question.

Worked Example — 1:2 Ratio HT Only

Q: 25.00 cm³ of the hydrochloric acid reacted with 23.50 cm³ of the 0.100 mol/dm³ barium hydroxide solution. The equation for the reaction is: 2HCl(aq) + Ba(OH)₂(aq) → BaCl₂(aq) + 2H₂O(l). Calculate the concentration of the hydrochloric acid in mol/dm³.4 MARKS

Citation: P1 Jun 2022 Q08.3 — verbatim AQA stem.

1

Moles Ba(OH)₂: n = c × v = 0.100 × (23.50 ÷ 1000) = 0.00235 mol.

2

Mole ratio: 2HCl : 1 Ba(OH)₂. So moles HCl = 2 × 0.00235 = 0.00470 mol.

3

Concentration HCl: c = moles ÷ v = 0.00470 ÷ (25.00 ÷ 1000) = 0.188 mol/dm³.

⚠️

Watch out: the coefficient of HCl is 2 (not 1). A 1:1 assumption gives 0.094 mol/dm³ — half the correct answer. Always read the equation coefficients before applying the ratio.

Examiner Alert — Unit Conversion HT Only

"Although most students convert the volume of a solution to dm³ for a titration calculation, far fewer did in this calculation, so their calculated number of moles was 1000 times the correct value... A common error was to multiply the moles and concentration together." — AQA Examiner Report, P1 Nov 2021 Q09.4

StepWrongRight

Molesn = c × v(cm³) → 1000× too bign = c × v(dm³) = c × v(cm³)/1000

Concentrationc = n ÷ v(cm³) → 1000× too smallc = n ÷ v(dm³) = n × 1000 / v(cm³)

Volumev = n × c (multiplication)v = n ÷ c (division)

Sanity Check

A typical titration concentration is around 0.01 to 1.0 mol/dm³. If your answer is 100 or 0.0001 mol/dm³, you've almost certainly missed a cm³ → dm³ conversion. Re-check before finalising.

Strong vs Weak Acids HT Only

Spec 4.4.2.6: this distinction is about the extent of ionisation in aqueous solution — a chemical property of the acid molecule itself, not how much is dissolved.

Strong Acid

Fully ionised in aqueous solution.

HCl(aq) → H⁺(aq) + Cl⁻(aq)

Single arrow — every molecule splits.

Examples: hydrochloric (HCl), nitric (HNO₃), sulfuric (H₂SO₄).

Weak Acid

Partially ionised in aqueous solution.

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

Equilibrium ⇌ — only a fraction ionised.

Examples: ethanoic, citric, carbonic.

The Mark-Scheme Phrase

Weak acid: "partially ionised in aqueous solution" (or "partially dissociated", or "in water"). Both clauses required for full marks at 2-mark questions: (1) partially ionised — (2) in aqueous solution.

Examiner Alert — Weak Acid Definition HT Only

"Over a half of students knew that a weak acid is one which is partially ionised, although many forgot to state that this was in aqueous solution. Some students incorrectly stated that an acid is ionising (ionises other species) rather than ionised (is itself ionised). Some incorrectly stated that it was H⁺ ions which are ionised or dissociated, rather than the acid molecules. The H⁺ ions are the result of the ionisation or dissociation." — AQA Examiner Report, P1 Jun 2024 Q09.1

Student wroteWhy it failed

"partially ionised" (no qualifier)Missing "in aqueous solution" — only 1 of 2 marks (Jun24 MS shows MP2 dependent on MP1).

"partially ionising"Active verb (transitive) — implies the acid ionises something else. The acid is itself ionised.

"H⁺ ions are ionised"H⁺ is the product of ionisation, not the species being ionised. The acid molecule ionises.

"a weak acid has higher pH"Defines via pH, not ionisation. Mark scheme rejects pH-based definitions (Jun23 ER).

Strong ≠ Concentrated HT Only

This is the single most-confused distinction in 4.4.2.6 — and the one the examiner reports flag every series. Strong/weak and concentrated/dilute are independent properties.

PropertyWhat it describesDepends on

Strong / weakThe extent of ionisation in aqueous solutionIdentity of the acid (a chemical property)

Concentrated / diluteThe amount of acid per unit volume of solutionHow much acid is dissolved (a physical property)

Four Combinations Possible

Concentrated strong acid — e.g. 1 mol/dm³ HCl. Lots of acid; all ionised.

Dilute strong acid — e.g. 0.001 mol/dm³ HCl. Very little acid; what's there is fully ionised.

Concentrated weak acid — e.g. concentrated ethanoic acid. Lots of acid; mostly un-ionised.

Dilute weak acid — e.g. dilute vinegar. Very little acid; mostly un-ionised.

Examiner Alert (P1 Jun 2022 Q08.1)

Q: "Explain why the pH of an acid depends on its strength AND its concentration." Mark scheme requires both: "the stronger the acid the lower the pH" (more ionisation → more H⁺) AND "the higher the concentration the lower the pH" (more acid per unit volume → more H⁺). ER: "Many students made incorrect statements trying to link strength to concentration, not recognising that they are entirely separate."

Practice 1 — Foundation

A student does a titration to find the volume of acid needed to neutralise an alkali. The student fills a burette with the acid. Give three more steps the student must do before adding the acid to the alkali from the burette. You should name any equipment used.3 MARKS

Citation: P1 Jun 2024 Q09.3 — verbatim AQA stem.

MP1

Use a (volumetric) pipette to add the alkali (to a conical flask). [Required first mark.]

MP2

Then any TWO from:

a

Into a conical flask (not a beaker — beaker ignored, not credited).

b

Add an indicator (named — phenolphthalein / methyl orange). Universal indicator NOT accepted.

c

Take the initial burette reading (read at the bottom of the meniscus, eye level).

d

Use a white tile under the conical flask.

★

Mark-economy answer: "Use a volumetric pipette to add the alkali to a conical flask; add 2-3 drops of phenolphthalein indicator; take the initial burette reading."

Practice 2 — HT Calculation HT Only

The student found that 25.0 cm³ of the sodium hydroxide solution was neutralised by 15.00 cm³ of the 0.0480 mol/dm³ ethanedioic acid solution. The equation for the reaction is: H₂C₂O₄ + 2 NaOH → Na₂C₂O₄ + 2H₂O. Calculate the concentration of the sodium hydroxide solution in mol/dm³.3 MARKS

Citation: P1 Jun 2021 (Nov21) Q09.5 — verbatim AQA stem.

1

Moles H₂C₂O₄: n = c × v = 0.0480 × (15.00 ÷ 1000) = 0.000720 mol.

2

Mole ratio: 1 H₂C₂O₄ : 2 NaOH. So moles NaOH = 2 × 0.000720 = 0.00144 mol.

3

Concentration NaOH: c = n ÷ v = 0.00144 ÷ (25.0 ÷ 1000) = 0.0576 mol/dm³.

★

Mark-scheme answer: 0.0576 mol/dm³ (allow 0.058 mol/dm³). Each step = 1 mark; ECF (error carried forward) is allowed if a wrong intermediate value is used correctly downstream.

+

g/dm³ extension: if the question asked for the concentration in g/dm³, multiply by Mr. Mr(NaOH) = 23 + 16 + 1 = 40. So 0.0576 × 40 = 2.304 g/dm³. (Spec 4.4.2.5 requires fluency in both units.)

Practice 3 — HT Analysis HT Only

lowest pH

1 MARK

A — 0.1 mol/dm³ ethanoic acid solution

B — 0.1 mol/dm³ hydrogen chloride solution

C — 1.0 mol/dm³ ethanoic acid solution

D — 1.0 mol/dm³ hydrogen chloride solution

Citation: P1 Jun 2021 (Nov21) Q09.2 — verbatim AQA stem.

★

Answer: D — 1.0 mol/dm³ hydrogen chloride solution.

1

Strength comparison: HCl is fully ionised (strong); ethanoic is 1% ionised (weak). For the same concentration, HCl has more H⁺ — lower pH.

2

Concentration comparison: 1.0 mol/dm³ has more acid per unit volume than 0.1 mol/dm³ — more H⁺ — lower pH.

3

Combined: the strongest acid at the highest concentration gives the lowest pH. That's D.

!

Both factors matter: 0.1 mol/dm³ HCl is more acidic than 1.0 mol/dm³ ethanoic acid (because the strength factor of 100× partial ionisation outweighs the 10× concentration difference). Strength and concentration are independent — and both reduce pH.

Exam-Ready Summary

RP2 Method (Foundation + HT)

Pipette alkali into conical flask · Phenolphthalein or methyl orange · Burette with acid · White tile.

Add acid slowly, swirl, drop-by-drop near endpoint, permanent colour change.

Repeat until concordant titres (within 0.10 cm³); mean of concordant only.

HT Calculation Steps

Step 1: moles of known = c × v (in dm³).

Step 2: apply the mole ratio from the balanced equation (read coefficients!).

Step 3: c of unknown = moles ÷ v (in dm³). Convert to g/dm³ by × Mr if asked.

Strong / Weak / Concentrated [HT]

Strong = fully ionised in aqueous solution. Examples: HCl, HNO₃, H₂SO₄.

Weak = partially ionised in aqueous solution. Examples: ethanoic, citric, carbonic.

Strong ≠ concentrated. Strength = property of the molecule. Concentration = how much per volume. Both reduce pH; they are independent.

Top Lost Marks

"Universal indicator" / "beaker" / "measuring cylinder" — all rejected (Jun24 Q09.3).

"Clear" instead of "colourless" for phenolphthalein endpoint (Jun23 Q05.3).

Forgetting to convert cm³ to dm³ in calculations — answer 1000× off (Nov21 Q09.4).

Mean of all 5 titres (including rough) instead of concordant only (Jun22 Q08.2).

"Partially ionising" (transitive) or "H⁺ ions are ionised" — wrong forms (Jun24 Q09.1).