Inorganic Qualitative Analysis: Quick Revision

A brief note on Inorganic Qualitative Analysis. Read it before a test. Try each question at the end before you open its answer.

Anions

• Dilute sulphuric acid: carbonate (carbon dioxide, milky lime water), sulphide (rotten eggs, black lead acetate paper), sulphite (choking smell, green dichromate paper), nitrite (brown fumes), acetate (vinegar).
• Concentrated sulphuric acid: chloride (white fumes with ammonia), bromide (reddish brown vapour), iodide (violet vapour), nitrate (brown fumes with copper).
• Brown ring test for nitrate: iron(II) sulphate, then concentrated acid slowly down the wall.
• Sulphate: barium chloride gives a white precipitate that stays in dilute hydrochloric acid.
• Silver halides: white, pale yellow, yellow.

Cations

• Zero $\text{NH}_{4}^{+}$: sodium hydroxide, ammonia gas. I $\text{Pb}^{2+}$: dilute HCl. II $\text{Cu}^{2+}$: hydrogen sulphide in acid.
• III $\text{Fe}^{3+}$, $\text{Al}^{3+}$: ammonium chloride and ammonium hydroxide. IV Zn, Ni, Co, Mn: hydrogen sulphide in ammonia.
• V Ca, Ba: ammonium carbonate. VI Mg: disodium hydrogenphosphate.
• Iron(III): Prussian blue, blood red with thiocyanate. Copper: deep blue with excess ammonia. Nickel: rosy red with DMG.
• Zinc and aluminium dissolve in excess sodium hydroxide. Calcium is brick red and barium apple green in a flame.

Why it works

• A precipitate forms when $Q > K_{sp}$.
• $[\text{S}^{2-}]$ is proportional to $1 / [\text{H}^+]^2$. In acid only copper sulphide forms. In ammonia, zinc, nickel, cobalt and manganese sulphides form too.
• Ammonium chloride lowers $[\text{OH}^-]$, so only iron(III) and aluminium hydroxides form in group III.

A worked example

Sodium chloride, 1.17 g, is treated with excess silver nitrate. What mass of silver chloride forms?

1. Moles. $\dfrac{1.17}{58.5} = 0.0200\ \text{mol}$.
2. Mass. $0.0200 \times 143.5 = 2.87\ \text{g}$.

Practice with new numbers

A program to try

The program follows a mixture of cations through the group scheme.

Mistakes to avoid

• Testing for ammonium after the group III reagent. It contains ammonium.
• Naming an anion from the first gas. Carbon dioxide and sulphur dioxide both turn lime water milky.
• Forgetting the dilute acid in the barium chloride test for sulphate.

Check yourself

1. Which gas gives a green dichromate paper?

2. Which cation gives a blood red colour with thiocyanate?

3. Why does hydrogen sulphide in acid precipitate copper sulphide only?

4. What do you see in the brown ring test?