Titration and Solubility

Titration is the single most examined technique on Paper 6 (apparatus names, burette readings to 0.05 cm³, concordant titres, indicator choice), and the solubility table that travels with it decides salt-preparation and ion-test answers across two other topics. None of it is conceptually hard; all of it is precision marking, where “beaker” instead of “conical flask” or an averaged rough titre costs real marks.

The apparatus and what each piece does (Core)

Apparatus	Job	Precision
Volumetric pipette (+ filler)	Delivers a fixed volume (25.0 cm³) into the flask	Very high, one fixed volume
Burette	Delivers a variable, measured volume	Reads to 0.05 cm³
Conical flask	Holds the solution being titrated; swirled without spilling	Not applicable
White tile	Sits under the flask to sharpen the colour change	Not applicable

The division of labour is the testable idea: pipette for the fixed volume, burette for the variable one. A measuring cylinder appears in wrong-answer lists every series: it reads only to about 1 cm³ and is never acceptable where a burette or pipette is named.

The technique, step by step (Core)

1. Rinse the pipette with the alkali, then pipette 25.0 cm³ of alkali into the conical flask.
2. Add 2-3 drops of indicator: thymolphthalein (blue in alkali → colourless) or methyl orange (yellow → red as acid is added). Universal indicator is wrong here: its gradual rainbow gives no sharp end-point.
3. Fill the burette with acid; remove the air bubble below the tap; record the initial reading to 0.05 cm³.
4. Run in acid with swirling. Near the end-point, add dropwise.
5. Stop at the first permanent colour change; record the final reading. Titre = final − initial.
6. Do a rough titration first, then repeat until two concordant titres (within 0.1-0.2 cm³) are obtained, and average only those.

Reading technique carries its own marks: read at eye level, from the bottom of the meniscus. The why-marks behind each step (rinsing, dropwise addition, discarding the rough) are classic Paper 6 “explain why” territory, and the calculation that follows (concentration from titre) belongs to Stoichiometry. Using the technique to manufacture a pure salt (including the repeat without indicator) is covered in Preparation of Salts.

The solubility rules (Core)

Learn the table as “everything is soluble except…” and the exceptions become the whole job:

Compound type	Rule
Sodium, potassium, ammonium salts	All soluble
Nitrates	All soluble
Chlorides	Soluble except silver and lead(II)
Sulfates	Soluble except barium, calcium and lead(II)
Carbonates	Insoluble except sodium, potassium, ammonium
Hydroxides	Insoluble except sodium, potassium, ammonium (calcium slightly)

Three jobs in the exam depend on this table. First, routing salt preparations: insoluble target → precipitation; soluble target → acid-based method or titration. Second, predicting precipitates: mixing silver nitrate with sodium chloride must give solid silver chloride, because both reactants are soluble and one product is not. Third, decoding ion tests: the white precipitates in the chloride and sulfate tests of qualitative analysis are AgCl and BaSO4, the same two exceptions doing diagnostic work.

Worked exam question

A student titrates 25.0 cm³ of aqueous sodium hydroxide with dilute hydrochloric acid. Burette readings (cm³): rough 24.6; titration 1: 23.95; titration 2: 24.05; titration 3: 23.50. (a) Name the piece of apparatus used to measure the 25.0 cm³ of sodium hydroxide. [1] (b) Explain which titres the student should use to calculate the average, and calculate it. [2] (c) Suggest a suitable indicator and state its colour change in this titration. [2]

Model answer: (a) (Volumetric) pipette (1). (b) Titrations 1 and 2: they are concordant, within 0.1 cm³ of each other (1); average = (23.95 + 24.05) ÷ 2 = 24.00 cm³ (1). (c) Methyl orange (1): yellow in the alkali, turning red/orange at the end-point as acid is added (1). (Thymolphthalein, blue → colourless, equally credited.)

Mark-by-mark: (a) rejects “measuring cylinder” outright. (b) requires the selection reason, concordance, not just the arithmetic; including the rough or titration 3 voids both marks. (c) needs the change stated in the direction this titration runs (alkali in the flask first).

The mistakes that cost marks

1. Averaging every titre, rough included. Concordant results only, and say the word “concordant”.
2. Universal indicator in a titration. No sharp end-point; choose methyl orange or thymolphthalein.
3. Burette volumes quoted to whole numbers. Readings go to 0.05 cm³: write 24.00, not 24.
4. Solubility guesses. “Lead nitrate is insoluble” contradicts the all-nitrates rule; check the table before predicting any precipitate.

How examiners want it phrased

Student wording	Mark-scheme wording
”Measure the alkali with a tube"	"Use a volumetric pipette to transfer 25.0 cm³ of sodium hydroxide"
"Do it again to check"	"Repeat until two concordant titres (within 0.1 cm³) are obtained"
"Add acid until it changes colour"	"Add acid dropwise near the end-point until the indicator just changes permanently"
"Salt won’t dissolve"	"Silver chloride is insoluble, so a white precipitate forms”

Precision words (pipette, burette, concordant, dropwise, meniscus) are the currency of this subtopic, and the parent overview is on the Acids, Bases and Salts pillar. If your practical answers describe the right experiment in the wrong vocabulary, one free 1-hour trial lesson with a Chemistry specialist will swap in the mark-scheme terms where they pay.

Test yourself

Answer with the precision words (pipette, concordant, dropwise), then click to check.

Q1 (2 marks). Explain why a rough titration is carried out first, and why its result is not included in the average.

Show answer

• the rough titration finds the approximate position of the end-point quickly [1] • it overshoots / is less accurate, so only the concordant accurate titres are averaged [1]

Q2 (3 marks). Two pairs of solutions are mixed: (a) lead(II) nitrate + sodium sulfate; (b) potassium chloride + sodium nitrate. Predict what is observed in each case, naming any precipitate.

Show answer

• (a) a white precipitate forms [1] • the precipitate is lead(II) sulfate, one of the three insoluble sulfates [1] • (b) no precipitate: all sodium, potassium and nitrate salts are soluble [1]

Q3 (2 marks). In a titration, explain why the alkali is placed in a conical flask rather than a beaker, and why the flask stands on a white tile.

Show answer

• the conical flask’s narrow neck lets the mixture be swirled without splashing or loss [1] • the white tile makes the indicator’s colour change at the end-point easier to see [1]