Writing Ionic Formulas, Given the ions

[BrainMan]

Homework Statement

Potassium chromate is an important compound of chromium. It is composed of K⁺
and CrO₄^2-. Write the formula of the compound.

Homework Equations

positive charge + negative charge = 0

The Attempt at a Solution

I know how to do this problem by cross switching the charges but I want to know how to do it by finding the ratio. So what I tried to do was say that it takes 8 K to equal the negative charge of the CrO₄ so the ration is 8:4. That means K₈ = CrO₄. Then I reduced and got K₂ = CrO. The correct answer is K₂ = CrO₄.

 

[Adithyan]

You seem to think that each oxygen has a double negative charge. This is not true. The TOTAL charge on CrO₄^2- is -2. So, consider K⁺ as A and CrO₄^2- as B and now try to balance the charges assuming charge on B is -2.

Hope this helps! :)
-Adithyan

 

[Borek]

In other words: CrO₄^2- means (CrO₄)^2-.

 

[BrainMan]

OK I get it now. Thanks!