Question

What is the metal oxidation number in the complex ion, `[Zn(OH)_4]^(2-)`?

Answer 1

`Zn(OH)_4^(2-)` is a complex ion in which the oxidation number of `Zn` is `+II`. Oxidation numbers of hydrogen (`+I`) and oxygen (`-II`) are standard.

Explanation:

How do we know this? Well, look at its formation reaction:

`Zn^(2+) + 4HO^(-) rarr [Zn(OH)_4]^(2-)`

Charge and mass are conserved, as in every chemical equation. The `[Zn(OH)_4]^(2-)` species is a complex ion, for which there are many examples.

Answer 2

Starting with the common grouping of a hydroxyl (OH) as `O^(-2), H^(+1)` for a net charge of -1. That gives a total of -4 charge for the four OH groups. If the whole molecule has a charge of -2, the Zn must be `Zn^(+2)`. Checking the Periodic Table, this is a highly likely oxidation state (giving up its 4s electrons).