The oxidation number of X in XO43- is

 The oxidation number of X in XO43- is

A. +1

B. +2

C. +3

D. +4

E. +5

The correct answer is E. +5

SOLUTION: 

Clue: equate the left hand side of the equation to the charge. In this case, the charge is negative 3. Remember that oxidation state of oxygen is -2. Substitute the value of oxygen into the formula.

 XO4=-3

X+(-2*4)=-3

X+(-8)=-3

X-8=-3

X=-3+8

X=+5

Hence the correct answer is E. +5

QUESTION SOURCE: NECO CHEMISTRY 2001 QUESTION 18