Q10 of 35 Page 1

Identify the following:

(i) Oxoanion of chromium which is stable in acidic medium.


(ii) The lanthanoid element that exhibits +4 oxidation state.

(i) Cr₂O72– (dichromate ion) is the oxoanion of chromium which is stable in acidic medium.


⇒ Oxoanions of the metals are compounds in which the metals form bonds and interact with the available oxygen atom.


⇒ Therefore, they exist in their highest oxidation states.


⇒ In the dichromate salts, the dichromate anion is represented as Cr₂O72– and the oxidation state of Chromium in it is:


Let the oxidation state of chromium is x


Oxidation state of oxygen is -2


∴ 2x + 7 × (-2) = -2


⇒ 2x -14 = -2


⇒ 2x = -2 +14


⇒ 2x = 12


⇒ x = 6


Hence, the oxidation state of chromium is +6


⇒ It acts as a powerful oxidizing agent.


Thus,, Cr2O₇2– is stable in acidic medium.


(ii) The lanthanoid elements that exhibits +4 oxidation state are:


Cerium (Ce), Praseodymium (Pr), Terbium (Tb), Neodymium (Nd) and Dysprosium (Dy)


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