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!

 

[DeeNos]

I would like to commend you for trying to understand the ratio method for writing ionic formulas. However, in this case, your approach is not entirely correct. The ratio method is used for covalent compounds, not ionic compounds. In ionic compounds, the charges of the ions must be balanced to achieve a neutral compound. In this case, the charges of K+ and CrO42- are already balanced, so there is no need to find a ratio. The correct way to write the formula for potassium chromate is K2CrO4, as you have mentioned. This is because the charges of K+ and CrO42- are 2+ and 2-, respectively, so they balance each other out. I hope this helps clarify any confusion.