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Preparation of Mohr’s salt and potash alum: NEET MCQs with answers

50 questions on this topic of Principles Related to Practical Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. During the preparation of Mohr's salt, which of the following precautions is specifically taken to prevent the oxidation of Fe²⁺ to Fe³⁺ by atmospheric oxygen?

  1. The solution is concentrated using an evaporating dish over an open intense flame.
  2. Excess concentrated nitric acid is added along with dilute sulphuric acid.
  3. The solution is concentrated only up to the crystallization point and not boiled for prolonged periods.
  4. The crystallization dish is continuously stirred with a glass rod while cooling.
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Answer: (C) The solution is concentrated only up to the crystallization point and not boiled for prolonged periods.

Prolonged boiling of the solution in the presence of air oxidizes ferrous ions (Fe²⁺) to ferric ions (Fe³⁺). Therefore, the solution is only heated gently until the crystallization point is attained.

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2. During the preparation of Mohr's salt, why is the concentrated solution left to cool slowly and undisturbed without any mechanical agitation or rapid chilling?

  1. Rapid cooling leads to decomposition into toxic ammonia and sulphur dioxide gases
  2. Disturbance prevents the hydration shell from attaching to the salt ions
  3. Mechanical agitation causes reduction of Fe²⁺ to metallic iron
  4. Undisturbed slow cooling promotes the growth of well-defined, larger crystals rather than microcrystals
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Answer: (D) Undisturbed slow cooling promotes the growth of well-defined, larger crystals rather than microcrystals

Slow, undisturbed cooling allows solute particles to deposit systematically on growing crystal faces, yielding distinct, large, and pure monoclinic crystals. Rapid cooling or agitation leads to rapid nucleation and tiny, ill-formed crystal aggregates with trapped mother liquor.

hard

3. What solid residue is left behind when pure potash alum crystals are subjected to prolonged, very strong heating in a crucible until no further change occurs?

  1. Al₂O₃ and K₂SO₄
  2. Pure metallic aluminium and potassium oxide
  3. Only K₂SO₄
  4. Al(OH)₃ and K₂SO₄
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Answer: (A) Al₂O₃ and K₂SO₄

On gentle heating, potash alum loses its water of crystallization to form burnt alum. On prolonged and strong heating, Al₂(SO₄)₃ decomposes into Al₂O₃ and SO₃ gas, while K₂SO₄ remains stable. Hence, the final solid residue consists of a mixture of Al₂O₃ and K₂SO₄.

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4. What is the primary role of adding a small volume of dilute sulphuric acid while dissolving aluminium sulphate during the synthesis of potash alum?

  1. To oxidize Al³⁺ ions to higher oxidation states
  2. To convert potassium sulphate into potassium bisulphate
  3. To act as a dehydrating agent to trigger rapid crystallization
  4. To prevent the precipitation of aluminium hydroxide caused by cationic hydrolysis
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Answer: (D) To prevent the precipitation of aluminium hydroxide caused by cationic hydrolysis

Aluminium sulphate undergoes hydrolysis in water to produce a gelatinous precipitate of aluminium hydroxide, Al(OH)₃, which makes the solution turbid. Adding dilute H₂SO₄ reverses and suppresses this hydrolysis, maintaining a clear solution.

easy

5. During the preparation of Mohr's salt, pure iron filings are reacted with dilute H₂SO₄. Which gas is evolved during this initial step, and what is its role in the reaction flask?

  1. Hydrogen gas; it provides a reducing atmosphere preventing oxidation of Fe²⁺ to Fe³⁺
  2. Sulphur dioxide gas; it acts as a bleaching agent for the solution
  3. Oxygen gas; it speeds up the dissolution of iron
  4. Ammonia gas; it neutralizes excess acid present in the solution
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Answer: (A) Hydrogen gas; it provides a reducing atmosphere preventing oxidation of Fe²⁺ to Fe³⁺

When Fe reacts with dilute H₂SO₄, FeSO₄ and H₂ gas are produced (Fe + H₂SO₄ → FeSO₄ + H₂↑). The evolution of hydrogen maintains a reducing atmosphere inside the flask, thereby preventing atmospheric oxygen from oxidizing Fe²⁺ to Fe³⁺.

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6. Which of the following statements correctly differentiates the behavior of Mohr's salt and potassium ferrocyanide in solution?

  1. Both salts give a positive Prussian blue test directly upon addition of potassium thiocyanate (KSCN)
  2. Potassium ferrocyanide produces 6 simple ions per formula unit, while Mohr's salt produces only 3 ions
  3. Mohr's salt retains its complex identity in aqueous solution, while potassium ferrocyanide completely ionizes into simple Fe²⁺ and CN⁻ ions
  4. Mohr's salt gives a positive test for Fe²⁺ with potassium ferricyanide, whereas potassium ferrocyanide does not dissociate to provide free Fe²⁺ ions
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Answer: (D) Mohr's salt gives a positive test for Fe²⁺ with potassium ferricyanide, whereas potassium ferrocyanide does not dissociate to provide free Fe²⁺ ions

Mohr's salt is a double salt which fully ionizes into Fe²⁺, NH₄⁺, and SO₄²⁻ ions in water, readily giving characteristic tests for Fe²⁺ (such as Turnbull's blue with K₃[Fe(CN)₆]). Potassium ferrocyanide, K₄[Fe(CN)₆], is a coordination complex that ionizes into 4K⁺ and [Fe(CN)₆]⁴⁻, keeping Fe²⁺ bound in the complex anion so it does not give direct tests for free Fe²⁺.

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7. To prepare 0.05 mol of Mohr's salt, FeSO₄·(NH₄)₂SO₄·6H₂O, what mass of ammonium sulphate [(NH₄)₂SO₄, molar mass = 132 g/mol] must be accurately weighed?

  1. 3.30 g
  2. 6.60 g
  3. 13.20 g
  4. 9.85 g
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Answer: (B) 6.60 g

The preparation of Mohr's salt requires an equimolar mixture of FeSO₄ and (NH₄)₂SO₄. For 0.05 mol of Mohr's salt, exactly 0.05 mol of (NH₄)₂SO₄ is needed. Mass = 0.05 mol × 132 g/mol = 6.60 g.

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8. When testing an aqueous solution of potash alum with chemical reagents, which pair of observations is correct?

  1. Forms a persistent white precipitate with excess NaOH that is insoluble upon further addition
  2. Forms a reddish-brown precipitate with aqueous NH₃, and gives no precipitate with BaCl₂
  3. Forms a white precipitate with BaCl₂ that readily dissolves in excess dil. HCl
  4. Forms a white gelatinous precipitate with excess NaOH that dissolves, and gives a white precipitate with BaCl₂ insoluble in dil. HCl
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Answer: (D) Forms a white gelatinous precipitate with excess NaOH that dissolves, and gives a white precipitate with BaCl₂ insoluble in dil. HCl

Potash alum completely dissociates into K⁺, Al³⁺, and SO₄²⁻ ions in water. Al³⁺ reacts with NaOH to yield gelatinous Al(OH)₃, which dissolves in excess NaOH forming soluble sodium aluminate [Al(OH)₄]⁻. SO₄²⁻ gives white BaSO₄ with BaCl₂ which is insoluble in dilute HCl.

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9. Which pair of reagents, when added separately to an aqueous solution of Mohr's salt, will confirm the presence of Fe²⁺ ions and SO₄²⁻ ions respectively?

  1. Nessler's reagent yielding a brown precipitate; and lead acetate yielding a black precipitate
  2. Potassium thiocyanate solution yielding a blood-red color; and silver nitrate solution yielding a yellow precipitate
  3. Potassium ferricyanide solution yielding a dark blue precipitate; and barium chloride solution yielding a white precipitate
  4. Sodium hydroxide yielding a reddish-brown precipitate; and dilute HCl yielding effervescence
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Answer: (C) Potassium ferricyanide solution yielding a dark blue precipitate; and barium chloride solution yielding a white precipitate

Fe²⁺ ions react with potassium ferricyanide, K₃[Fe(CN)₆], to give Turnbull's blue (potassium iron(II) hexacyanoferrate(III)). SO₄²⁻ ions react with BaCl₂ solution to form a white precipitate of BaSO₄ which is insoluble in dilute HCl.

easy

10. In the laboratory preparation of Mohr's salt, what is the stoichiometric mole ratio in which ferrous sulphate heptahydrate (FeSO₄·7H₂O) and ammonium sulphate ((NH₄)₂SO₄) are mixed?

  1. 1 : 2
  2. 2 : 1
  3. 1 : 6
  4. 1 : 1
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Answer: (D) 1 : 1

Mohr's salt is an equimolar double salt with the formula FeSO₄·(NH₄)₂SO₄·6H₂O. Therefore, ferrous sulphate and ammonium sulphate must be mixed in a 1 : 1 stoichiometric mole ratio.

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11. What is the total number of water molecules of crystallization present per formula unit in the traditional combined formula of potash alum, K₂SO₄·Al₂(SO₄)₃·xH₂O?

  1. 6
  2. 24
  3. 12
  4. 48
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Answer: (B) 24

The traditional double salt formula of potash alum is written as K₂SO₄·Al₂(SO₄)₃·24H₂O. Therefore, it contains 24 molecules of water of crystallization per formula unit. Its equimolar simplified formula is KAl(SO₄)₂·12H₂O.

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12. Given below are two statements regarding double salts: Statement I: An aqueous solution of Mohr's salt gives a positive test for Fe²⁺, NH₄⁺, and SO₄²⁻ ions. Statement II: Unlike complex salts, double salts completely dissociate into their constituent simple ions in aqueous solution. In the light of the above statements, choose the most appropriate answer:

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
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Answer: (A) Both Statement I and Statement II are correct.

Mohr's salt is a double salt and completely dissociates into Fe²⁺, NH₄⁺, and SO₄²⁻ ions in water, showing positive tests for all constituent ions. Complex salts retain their identity and do not dissociate completely into simple constituent ions.

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