Question

A student is given an unknown aqueous solution that contains either the divalent ion or trivalent ion of a certain metal (that is, the solution either contains metal(II) ions or metal(III) ions of the same metal). The student was also given a dilute solution of potassium permanganate which had a vivid purple color. Which of the following might be an appropriate way to ascertain which unknown solution the student was given?

Addition of the permanganate solution to the unknown solution would colorimetrically indicate whether metal(II) is oxidized to metal(III). If the unknown solution contained the metal(II) ion, the purple color of the solution would get paler. If the unknown contained the metal(III) ion, the purple color of the solution would remain the same.

Addition of the permanganate solution to the unknown solution would colorimetrically indicate whether metal(III) is reduced to metal(II). If the unknown contained the metal(III) ion, the purple color of the solution would get paler. If the unknown contained the metal(II) ion, the purple color of the solution would get darker.

Addition of the permanganate solution to the unknown solution would colorimetrically indicate whether metal(II) is oxidized to metal(III). If the unknown solution contained the metal(III) ion, the purple color of the solution would get paler. If the unknown contained the metal(II) ion, the purple color of the solution would remain the same.

Addition of the permanganate solution to the unknown solution would colorimetrically indicate whether metal(II) is oxidized to metal(III). If the unknown solution contained the metal(II) ion, the purple color of the solution would get darker. If the unknown contained the metal(III) ion, the purple color of the solution would remain the same.

Addition of the permanganate solution to the unknown solution would colorimetrically indicate whether metal(III) is reduced to metal(II). If the unknown solution contained the metal(III) ion, the purple color of the solution would get darker. If the unknown contained the metal(II) ion, the purple color of the solution would remain the same.

Answer 1

The unknown solution contained either the divalent metal ion (oxidation state +2) or the trivalent metal ion (+3). The student was given a dilute solution of potassium permanganate (KMnO₄) to identify the metal ion.

KMnO₄ contained the permanganate, MnO₄^- ion which has manganese (Mn) in its highest oxidation state (+7). Therefore, Mn can no further be oxidized, i.e, MnO₄^- can only be reduced and therefore, acts as the oxidizing agent.

Consequently, MnO₄^- can only oxidizer the metal to the higher oxidation state. Since +3 is the highest oxidation state of the metal, hence, permanganate will oxidize the divalent metal to its trivalent oxidation state and itself get reduced. The +7 state of Mn is colored, i.e, only MnO₄^- is intensely purple with the lower oxidation states of Mn being pale colored. Hence, the purple color of the solution will fade due to the reaction of permanganate with the metal. Therefore, the first statement is the correct statement.