(A) +2 (B) +4 (C) +6 (D) +8
Solution: Let's first go over some rules for determining the oxidation number.
Rules for determining oxidation numbers:
Free, neutral elements have an oxidation number of 0.
Group 1 metals have an oxidation number of +1 (can be found on the Periodic Table)
Group 2 metals have an oxidation number of +2
Oxygen is -2 in all compounds other than peroxide (H2O2)
Hydrogen is +1 unless it is bonded to a metal
The sum of oxidation numbers in a neutral molecule is 0
The sum of oxidation numbers in a charged molecule (ion) is equal to the charge
Now, we can look at our molecule MoO4^2-
Let's say the oxidation number of Mo is X.
The overall charge of the molecule is 2-. The oxidation number of oxygen is -2.
Putting it all together, we can make an equation X+ (4*-2)=-2
X-8= -2
X = +6 (C)
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