Chemistry · Lesson 17

Qualitative analysis & volumetric analysis (titration)

JAMBWAECNECO

What the syllabus asks

JAMB: Qualitative tests for ions and gases; acid–base titration and simple calculations

WAEC: Qualitative analysis: tests for cations (NH₄⁺, Ca²⁺, Pb²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺, Zn²⁺), anions (CO₃²⁻, SO₃²⁻, SO₄²⁻, Cl⁻, NO₃⁻, S²⁻) and gases (H₂, O₂, CO₂, NH₃, Cl₂, HCl, SO₂, H₂S, NO₂); Quantitative (volumetric) analysis: acid–base titrations, indicators, standard solutions, calculations of concentration, molar mass, purity, water of crystallisation

NECO: Qualitative analysis: tests for cations (NH₄⁺, Ca²⁺, Pb²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺, Zn²⁺), anions (CO₃²⁻, SO₃²⁻, SO₄²⁻, Cl⁻, NO₃⁻, S²⁻) and gases (H₂, O₂, CO₂, NH₃, Cl₂, HCl, SO₂, H₂S, NO₂); Quantitative (volumetric) analysis: acid–base titrations, indicators, standard solutions, calculations of concentration, molar mass, purity, water of crystallisation

What you will learn

Perform and describe tests for common gases.
Identify common cations using NaOH(aq), NH₃(aq) and flame tests.
Identify common anions using standard tests.
Describe the apparatus and procedure of acid–base titration and choose an indicator.
Calculate concentrations, molar masses, percentage purity and water of crystallisation from titration data.
Record and present results in the WAEC practical format.

Key points

Qualitative: what is present; quantitative: how much.
Gas tests: H₂ pop; O₂ relights splint; CO₂ lime water milky; NH₃ red litmus blue; Cl₂ bleaches; SO₂ dichromate green; H₂S lead acetate black.
NaOH: Cu²⁺ blue; Fe²⁺ green; Fe³⁺ red-brown; Zn²⁺, Al³⁺, Pb²⁺ white soluble in excess; Ca²⁺ white insoluble.
NH₃: Zn²⁺ dissolves in excess; Cu²⁺ deep blue solution; Al³⁺, Pb²⁺ insoluble.
Anions: CO₃²⁻ acid/CO₂; SO₄²⁻ HCl + BaCl₂ white; Cl⁻ HNO₃ + AgNO₃ white; NO₃⁻ brown ring.
Titration: burette 2 d.p., concordant within 0.10 cm³; rinse with the solution.
Methyl orange for strong acid/weak base; phenolphthalein for weak acid/strong base.
CₐVₐ/C_bV_b = nₐ/n_b; g dm⁻³ = mol dm⁻³ × M.

Start here: Qualitative and quantitative analysis

When the NAFDAC laboratory checks whether a drug is genuine, it asks two questions: what is in it, and how much? These are the two branches of chemical analysis.

Qualitative analysis identifies the substances, ions or gases present, by colour changes, precipitates, smells and gas tests.
Quantitative analysis measures amounts, for example by titration (volumetric analysis) or by weighing (gravimetric analysis).

In the WAEC practical, you will usually be given a salt labelled "C" or "D" and asked to carry out tests, record Test, Observation and Inference in a table, and identify ions. You will also be given an acid and a base and asked to titrate and calculate.

Laboratory safety

Wear goggles; never taste chemicals.
Smell gases by wafting gently, not sniffing directly.
Add concentrated acid to water, not water to acid.
Heat test tubes pointing away from people.

Worked example 17.1

Classify: (a) finding the concentration of vinegar; (b) testing tap water for chloride ions.

(a) measures an amount: quantitative.
(b) identifies an ion: qualitative.

(a) quantitative; (b) qualitative

Try it yourself. What three column headings are used in WAEC qualitative analysis tables?

Show solution

Test, Observation, Inference.

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Practice questions

1. A precipitate that is white and dissolves in excess NaOH(aq) suggests

Zn²⁺ or Al³⁺ or Pb²⁺
Fe³⁺
Cu²⁺
Fe²⁺

Show answer

A: Zn²⁺ or Al³⁺ or Pb²⁺
These hydroxides are amphoteric.

2. A blue precipitate with NaOH(aq) shows the presence of

Fe²⁺
Zn²⁺
Ca²⁺
Cu²⁺

Show answer

D: Cu²⁺
Cu(OH)₂ is blue.

3. A reddish-brown precipitate with NaOH(aq) shows

Cu²⁺
Al³⁺
Fe³⁺
Fe²⁺

Show answer

C: Fe³⁺
Fe(OH)₃.

4. A dirty-green precipitate with NaOH(aq) shows

Pb²⁺
Fe²⁺
Fe³⁺
Cu²⁺

Show answer

B: Fe²⁺
Fe(OH)₂.

5. Chloride ions are detected with acidified silver nitrate, which gives a

white precipitate
yellow precipitate
blue solution
brown gas

Show answer

A: white precipitate
AgCl.

Key words

Qualitative analysis: Identifying the substances or ions present.
Quantitative analysis: Measuring the amount of substance present.
Precipitate: An insoluble solid formed in a solution.
Reagent: A substance used to bring about a test reaction.
Standard solution: A solution of accurately known concentration.
Titration: Adding one solution from a burette to another until reaction is just complete.
Titre: The volume of solution added from the burette.
End point: The point at which the indicator changes colour.
Equivalence point: The point at which reactants have reacted exactly in the mole ratio.
Indicator: A substance that changes colour with pH.
Molar concentration: Moles of solute per dm³ (mol dm⁻³).
Mass concentration: Grams of solute per dm³ (g dm⁻³).

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