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Ionic Equilibrium question

2021 · 18 Mar · Shift 1 · Q14
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  5. /2021 · 18 Mar · Shift 1 · Q14

Ionic Equilibrium question

2021 · 18 Mar · Shift 1 · Q14

JEE MainChemistryIonic EquilibriumMCQ+4 / −1
Given below are two statements : One is labelled as Assertion A and the other labelled as reason R Assertion A : During the boiling of water having temporary hardness, Mg(HCO3)2Mg(HCO_3)_2Mg(HCO3​)2​ is converted to MgCO3MgCO_3MgCO3​. Reason R : The solubility product of Mg(OH)2Mg(OH)_2Mg(OH)2​ is greater than that of MgCO3MgCO_3MgCO3​. In the light of the above statements, choose the most appropriate answer from the options given below :
  1. A
    Both A and R are true but R is not the correct explanation of A
  2. B
    A is true but R is false
  3. C
    Both A and R are true and R is the correct explanation of A
  4. D
    A is false but R is true
View written solutionFree

Correct answer: QUESTION APPEARS INCORRECT BECAUSE BOTH A AND R ARE FALSE. IF FORCED, NO OPTION MATCHES EXACTLY.

  1. Check Assertion A

Temporary hardness due to magnesium bicarbonate is removed on boiling.

A common school-level reaction is: Mg(HCO3)2→boilingMg(OH)2↓+2CO2↑Mg(HCO_3)_2 \xrightarrow{\text{boiling}} Mg(OH)_2 \downarrow + 2CO_2 \uparrowMg(HCO3​)2​boiling​Mg(OH)2​↓+2CO2​↑

Magnesium bicarbonate does not chiefly give precipitated MgCO3MgCO_3MgCO3​ on boiling water; instead, due to hydrolysis, Mg(OH)2Mg(OH)_2Mg(OH)2​ precipitates.

So, Assertion A is false.

  1. Check Reason R

Reason says: the solubility product of Mg(OH)2Mg(OH)_2Mg(OH)2​ is greater than that of MgCO3MgCO_3MgCO3​.

Typical values:

  • Ksp(Mg(OH)2)∼10−11K_{sp}(Mg(OH)_2) \sim 10^{-11}Ksp​(Mg(OH)2​)∼10−11
  • Ksp(MgCO3)∼10−5 to 10−6K_{sp}(MgCO_3) \sim 10^{-5} \text{ to } 10^{-6}Ksp​(MgCO3​)∼10−5 to 10−6

Hence, Ksp(Mg(OH)2)<Ksp(MgCO3)K_{sp}(Mg(OH)_2) < K_{sp}(MgCO_3)Ksp​(Mg(OH)2​)<Ksp​(MgCO3​)

So the given reason statement, as written, is false.

  1. Conclusion from chemistry

From direct chemical facts:

  • Assertion A = False
  • Reason R = False

None of the listed options exactly matches (A false,R false)(A\text{ false}, R\text{ false})(A false,R false).

  1. Compare with stored answer

Stored correct answer is D, i.e.

  • Assertion A is false
  • Reason R is true

But the reason is not true because the actual relation is: Ksp(Mg(OH)2)<Ksp(MgCO3)K_{sp}(Mg(OH)_2) < K_{sp}(MgCO_3)Ksp​(Mg(OH)2​)<Ksp​(MgCO3​)

Therefore, I do not agree with the stored answer.

  1. Most likely issue in the question

If the reason had been written as: "The solubility product of Mg(OH)2 is smaller than that of MgCO3"\text{"The solubility product of } Mg(OH)_2 \text{ is smaller than that of } MgCO_3\text{"}"The solubility product of Mg(OH)2​ is smaller than that of MgCO3​" then Reason would be true, and option D would make sense.

But with the statement exactly as given, the correct logical evaluation is:

  • A false
  • R false

Since that option is missing, the question appears flawed.

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