# Question 09e54

Jul 3, 2015

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, ${K}_{2} C {r}_{2} {O}_{7}$, 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 $\text{+1}$. On the other hand, oxygen is a nonmetal that almost always has an oxidation state of $\text{-2}$.

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 $\text{+6}$.