Below is a question from the 2017 USNCO:

The concentration of which approximately 0.01M solution could be most accurately determined by a visible spectrophotometer (or colorimeter)?

(A) $\ce{Mn(NO3)2}$

(B) $\ce{Co(NO3)2}$

(C) $\ce{Zn(NO3)2}$

(D) $\ce{Pb(NO3)2}$

The answer sheet states B is the correct answer.

I researched the colours of the ions in solution, and found:

  • Mn is pink
  • Co is pink
  • Zn is clear
  • Pb is clear

I was unsure how to progress from there.

How should I approach this question in an exam if there was no reference book?

  • $\begingroup$ Can you find any information about the faintness/brightness of the color of given ions? I don't know if there is a standard scale for this. $\endgroup$ – Tyberius Dec 29 '17 at 2:02
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    $\begingroup$ This really strikes me as a question where you must just know the answer rather than being able to rationalize it. $\endgroup$ – MaxW Dec 29 '17 at 4:01
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    $\begingroup$ This is a poor question, and can only be answered properly if only one species is coloured in the visible part of the spectrum (also not defined in the question). How can one determine which can be 'most accurately' determined without knowing something about the instrument to be used. Just because one species may absorb more strongly than another does not mean that it can be measured more accurately: solutions can absorb too much as well as too little. $\endgroup$ – porphyrin Dec 31 '17 at 18:08

An aqueous solution of $\ce{Mn^2+}$ is only faintly pink, whereas that of $\ce{Co^2+}$ is much more colored:

some ionic solutions; image source http://staff.buffalostate.edu/nazareay/che112/3d-22.JPG

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    $\begingroup$ Thank you. Is there any way of determining this in an exam situation? $\endgroup$ – George Tian Dec 29 '17 at 2:52
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    $\begingroup$ It's probably a good idea to: 1. Cite the source of the image; 2. Explain the intensity of the color based on electron transfer of the complexes. $\endgroup$ – andselisk Dec 29 '17 at 2:57
  • $\begingroup$ @GeorgeTian, you might be able to say that $\ce{Zn^2+}$ should be colorless because it has filled orbitals, but I'm not aware of a simple way of differentiating between the colors of the other options. $\endgroup$ – a-cyclohexane-molecule Dec 29 '17 at 3:02
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    $\begingroup$ @andselisk, right now I have the image source in what I think is a markdown field for the image description (you'll see it if you try to edit my post), but I'm not sure if it is accessible directly. Do you suggest putting the source explicitly underneath the image? Regarding your other point, I'm not qualified to provide such an explanation, unfortunately. At the USNCO level this is simply a matter of memorization. $\endgroup$ – a-cyclohexane-molecule Dec 29 '17 at 3:13
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    $\begingroup$ Unfortunately some sort of unjustified rationalization is often used to explain facts. $\endgroup$ – MaxW Dec 29 '17 at 3:59

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