Question

Calculate the oxidation number of underlined elements in the following species. CO₂, Cr₂O₇^2-, Pb₃O₄, PO₄^3- 

Answer 1

1. C in CO₂. Let oxidation number of C be x. Oxidation number of each O atom = -2. Sum of oxidation number of all atoms = x+2 (-2) ⇒ x - 4. 

As it is neutral molecule, the sum must be equal to zero. 

∴ x-4 = 0 (or) x = + 4 

2. Cr in Cr₂O₇^2-. Let oxidation number of Cr = x. Oxidation number of each oxygen atom =-2. Sum of oxidation number of all atoms

2x + 7(-2) = 2x - 14

Sum of oxidation number must be equal to the charge on the ion. 

Thus, 2x - 14 = -2

2x = +12

x = 12/2

x = 6