# Writing Ionic Formulas, Given the ions

1. May 29, 2014

### BrainMan

1. The problem statement, all variables and given/known data
Potassium chromate is an important compound of chromium. It is composed of K+
and CrO42-. Write the formula of the compound.

2. Relevant equations
positive charge + negative charge = 0

3. 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.

2. May 30, 2014

You seem to think that each oxygen has a double negative charge. This is not true. The TOTAL charge on CrO42- is -2. So, consider K+ as A and CrO42- as B and now try to balance the charges assuming charge on B is -2.

Hope this helps! :)

3. May 30, 2014

### Staff: Mentor

In other words: CrO42- means (CrO4)2-.

4. May 30, 2014

### BrainMan

OK I get it now. Thanks!