Question

Question #09e54

Answer 1

You go by the oxidation numbers of potassium and oxygen.

Explanation:

You're actually looking for oxidation number, not charge.

The oxidation state of chromium in potassium dichromate, `K_2Cr_2O_7`, can be determine with two things in mind

• the oxidation states of potassium and oxygen;
• thefact that the compound is neutral.

Potassium is a group 1 metal, which means that its oxidation state is always `"+1"`. On the other hand, oxygen is a nonmetal that almost always has an oxidation state of `"-2"`.

So, start by writing down what you know

`stackrel(color(blue)(+1))(K_2) stackrel(color(green)(?)) (Cr_2) stackrel(color(blue)(-2))(O_7)`

The compound is neutral, which means that the oxidation states of all atoms that make up the compound must add up to give zero.

Since you have 2 potassium, 2 chromine, and 7 oxygen atoms, you'll get

`2 * (color(blue)(+1)) + 2 * color(green)("?") + 7 * color(blue)(("-2")) = 0`

`2 + 2? - 14 = 0 => ? = (14-2)/2 = color(green)("+6")`

The oxidation state of chromium in potassium dichromate is indeed `"+6"`.