# USNCO Question - Determining solute using spectrophotometer

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

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?

• 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.
– Tyberius
Dec 29, 2017 at 2:02
• This really strikes me as a question where you must just know the answer rather than being able to rationalize it.
– MaxW
Dec 29, 2017 at 4:01
• 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. Dec 31, 2017 at 18:08

An aqueous solution of $\ce{Mn^2+}$ is only faintly pink, whereas that of $\ce{Co^2+}$ is much more colored:
• @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. Dec 29, 2017 at 3:02