Question

Q.) Oxidation number of P in PO4³, of S in SO4² and that of Cr in Cr₂O7² are respectively:

(a) +3, +6 and +5
(b) +5, +3 and +6
(c) +3, +6 and +6
(d) +5, +6 and +6










​

Answer 1

The correct option is (c), +3, +6 and +6.

In a compound, the sum of the oxidation numbers of all the atoms must be equal to the overall charge on the compound. In the case of PO4³-, the overall charge is -3, so the sum of the oxidation numbers of P and O must be -3. We know that the oxidation number of O is -2, so the oxidation number of P must be +3, which is option (c).

Similarly, in the case of SO4²-, the overall charge is -2, so the sum of the oxidation numbers of S and O must be -2. We know that the oxidation number of O is -2, so the oxidation number of S must be +6, which is option (c).

Finally, in the case of Cr₂O7²-, the overall charge is -2, so the sum of the oxidation numbers of Cr and O must be -2. We know that the sum of the oxidation numbers of the O atoms is -14, so the sum of the oxidation numbers of the Cr atoms must be +12. Since there are two Cr atoms, each must have an oxidation number of +6, which is option (c).