MCQMediumJEE 2023Ligands & Coordination Number

JEE Chemistry 2023 Question with Solution

**1L1 \, \text{L}, 0.02M0.02 \, \text{M} solution of [Co(NH3)5SO4]Br[Co(NH_3)_5SO_4]Br is mixed with 1L1 \, \text{L}, 0.02M0.02 \, \text{M} solution of [Co(NH3)5Br]SO4[Co(NH_3)_5Br]SO_4. The resulting solution is divided into two equal parts (X) and treated with excess AgNO3AgNO_3 solution and BaCl2BaCl_2 solution respectively as shown below:

1L1 \, \text{L} Solution (X) + AgNO3AgNO_3 solution (excess) Y\rightarrow Y

1L1 \, \text{L} Solution (X) + BaCl2BaCl_2 solution (excess) Z\rightarrow Z

The number of moles of Y and Z respectively are:**

  • A

    0.020.02, 0.020.02

  • B

    0.010.01, 0.010.01

  • C

    0.020.02, 0.010.01

  • D

    0.010.01, 0.020.02

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: 1L1 \, \text{L} of 0.02M0.02 \, \text{M} solution of [Co(NH3)5SO4]Br[Co(NH_3)_5SO_4]Br is mixed with 1L1 \, \text{L} of 0.02M0.02 \, \text{M} solution of [Co(NH3)5Br]SO4[Co(NH_3)_5Br]SO_4. The final solution is divided into two equal parts.

Find: The moles of precipitates Y and Z formed with excess AgNO3AgNO_3 and excess BaCl2BaCl_2 respectively.

Each original solution contains

0.02×1=0.02 mol0.02 \times 1 = 0.02 \text{ mol}

of the coordination compound.

After mixing, the total volume becomes 2L2 \, \text{L}. Since the mixture is divided into two equal parts, each part contains half of each solute. Therefore, one part contains 0.010.01 mol of [Co(NH3)5SO4]Br[Co(NH_3)_5SO_4]Br and 0.010.01 mol of [Co(NH3)5Br]SO4[Co(NH_3)_5Br]SO_4.

For the first half:

[Co(NH3)5SO4]Br+AgNO3AgBr\left[ \text{Co}(NH_3)_5SO_4 \right]Br + \text{AgNO}_3 \rightarrow \text{AgBr} \downarrow 0.01 molexcess0.01 mol0.01 \text{ mol} \qquad \text{excess} \qquad 0.01 \text{ mol}

So, Y = 0.010.01 mol.

For the second half:

[Co(NH3)5Br]SO4+BaCl2BaSO4\left[ \text{Co}(NH_3)_5Br \right]SO_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4 \downarrow 0.01 molexcess0.01 mol0.01 \text{ mol} \qquad \text{excess} \qquad 0.01 \text{ mol}

So, Z = 0.010.01 mol.

Thus, the solution concludes that the correct answer is 0.010.01 moles for both Y and Z. This corresponds to option C as stated in the solution, although the listed options show 0.01,0.010.01, 0.01 under option B.

Common mistakes

  • Assuming the full mixed solution is used in each test. This is wrong because the resulting solution is divided into two equal parts. Use only half of the total moles in each reaction.

  • Treating coordinated ions as free precipitating ions. This is wrong because only the counter ion outside the coordination sphere immediately gives precipitate with the reagent.

  • Ignoring which ion is outside the coordination sphere in each complex. In [Co(NH3)5SO4]Br[Co(NH_3)_5SO_4]Br, BrBr^- is outside, while in [Co(NH3)5Br]SO4[Co(NH_3)_5Br]SO_4, SO42SO_4^{2-} is outside. Identify the counter ion before predicting precipitate formation.

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