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My original question is one that is stated in the title, and I also have some side questions too.

I came across this question where you have a CuSO4 aqueous solution. Then you take an Ammonia aqueous solution and mix those two solutions. Then you take some Trichloromethane, and you mix it with the above mixed solution. The objective of the question is to determine the ratio of Cu2+ : NH3 in the formed complex.

So, for example, my assumption is that when CuSO4 and NH3 aqueous solutions are mixed, the Cu2+ ions form complex with Ammonia as [Cu(NH3)4(H2O)2]2+. According to the question there is excess Ammonia so, when you put Trichloromethane, Ammonia distributes between Trichloromethane and Water.

My first question is, does Cu2+ ions also distribute in between Trichloromethane and water? If it is, are there [Cu(NH3)4(H2O)2]2+ distributed between these two layers? (more precisely, are there [Cu(NH3)4(H2O)2]2+ or Cu2+ ions in Trichloromethane layer?)

For my second question consider following equilibrium.

\begin{align} \ce{[Cu(H2O)_6]^2+ + 2NH3 &<=> [Cu(OH)_2(H2O)_6] + 2NH4^+}\\ \ce{[Cu(OH)_2(H2O)_4] + 4NH3 &<=> [Cu(NH3)_4(H2O)_2]^2+ +2OH- +2H2O}\\ \end{align}

In the question, it uses titration to measure the NH3 amount dissolved in each layer. It uses HCl for the titration. So, as these are equilibriums, is it OK to use this titration, because when HCl react with NH3, the above equilibriums reverse bias their equilibrium to release more Ammonia into the system?

A guide with the thought process about this question would be helpful.

Sorry in advance if my English is hard to understand. I couldn't find the English version of this question. I will a post photo of the original question, which is written in Sinhala, so if you need you can translate it from Google. enter image description here

Also, below is a slightly modified accurate translation than Google translate that I made.

25.0 cm3 of an aqueous ammonia solution was mixed with 25.0 cm3 of a solution of 0.10 mol dm-3 CuSO4. The resulting deep blue solution was stirred well with about 50 cm3 of trichloromethane and allowed to equilibrate at a certain temperature. 31.9 cm3 of 0.02 mol dm-3 HCl solution was required to neutralize the ammonia at 25.0 cm3 of the organic layer. 20.8 cm3 of 1.00 mol dm-3 HCl solution was required to neutralize 25.0 cm3 of the dark blue aqueous ammonia layer. Calculate the Cu2+: NH3 ratio of the complex formed by molecular association of NH3 and Cu2+.

The distribution constant for NH3, between water and organic layer is 25.

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  • $\begingroup$ I would like to understand the stuff. You state that the blue complex of copper and ammonia contains $4$ $\ce{NH3}$ for one copper atom. So why do you try to calculate the ratio $\ce{NH3/Cu^{2+}}$ by titration if you know that this ratio is $4$ ? $\endgroup$
    – Maurice
    May 17, 2023 at 15:35
  • $\begingroup$ @Maurice OP doesn't really get the relevant equilibrium - there won't be any charged complex in the organic phase. $\endgroup$
    – Mithoron
    May 17, 2023 at 16:40
  • $\begingroup$ @Maurice sorry for not clarifying. I used it as an example to show there is an equilibrium. The ratio 4, was taken as an example as this is the most common equilibrium. $\endgroup$ May 17, 2023 at 16:57
  • $\begingroup$ By ligands, there are meant molecules or ions attached to the central ion/atom, not the whole complex ion. $\endgroup$
    – Poutnik
    May 18, 2023 at 9:01
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    $\begingroup$ Chem+Math Expression formatting reference: MathJax Basics / Chem+Math expressions/formulas/equations / Upright vs italic / Math SE Mathjax tutorial // MathJax is preferred not to be used in CH SE Q titles. $\endgroup$
    – Poutnik
    May 19, 2023 at 9:59

1 Answer 1

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[OP] My first question is, does $\ce{Cu^2+}$ ions also distribute in between Trichloromethane and water?

No, the charged species will stay in the aqueous layer.

[OP] So, as these are equilibriums, is it OK to use this titration, because when $\ce{HCl}$ react with $\ce{NH3}$, the above equilibriums reverse bias their equilibrium to release more Ammonia into the system?

Yes, that is the point of the titration of the aqueous layer. There is some free ammonia, and some ammonia released from the complex as you titrate with HCl.

[Instructions] The distribution constant for NH3, between water and organic layer is 25.

The titration of the organic layer with HCl gives you the concentration of ammonia in the organic layer. The concentration of (free) ammonia in water will be 25-times higher. You have to subtract the amount of free ammonia in water from the measured amount in the aqueous layer to figure out the amount that was complexed with copper.

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  • $\begingroup$ I approached the question with the wrong path. I thought titration was used to measure free ammonia in aqueous layer. Thanks for the explanation and could you also clarify what "[OP]" in your answer means. Thank you. $\endgroup$ May 19, 2023 at 20:25
  • $\begingroup$ @donthababakka OP is original poster, so that’s you. A bit of Stackexchange jargon. $\endgroup$
    – Karsten
    May 19, 2023 at 23:09

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