[delusioned]

The compound, Na3CrO4 (does this exist? i'm not sure if it does exist or is it a typo-error with my question paper), is an unstable dark green solid. The addition of dilute sulphuric acid to this produces a solution containing chromium (III) ions and dichromate (VI) ions, Cr2O7^2-. When a 1.00g sample of the impure green solid was reacted in this way, the Cr(VI) in the resulting solution reacted with an excess of potassium iodide to liberate 5.0 x 10^-3 moles of iondine, I2.

(a) Calculate the oxidation state of chromium in Na3CrO4, and construct a balance equation for its reaction with dilute acid.

I need some help with this question,
Firstly, does the compound actually exist? What I know is that CrO4 has a oxidation state of -2, doesn't it mean that Sodium Chromate is Na2CrO4 then?
Secondly, any idea what products are formed in the equation?

I need to get my assignment done soon, please help.
Thanks in advance.

[Borek]

You are told everything you need. Na₃CrO₄ is not a typo - and it should be fairly ease for you to calculate charge on CrO₄. Na₃CrO₄ is neutral. What is Na charge?

[delusioned]

Yes, I have managed to get the oxidation state of Cr.
But still thinking of the balanced equation,
below is what I've thought of thus far, however it seems impossible..

Na3CrO4 + H2SO4 -> Na2Cr2O7 + Cr2(SO4)3 + H2O

any hints regarding this?

[Borek]

Try to balance net ionic reaction. Start outlining half reactions - you will have the same reactant, but different products. Such a process is called disproportionation.

To be honest - I have not heard about Na₃CrO₄, but I know that compounds with Cr(V) do exist.

[delusioned]

CrO4^2- + 8H+  +3e  ->   Cr^3+  + 4H2O     
2CrO4^2  + 2H+           ->   Cr2O7^2-   + H2O

These? (not to sure if I'm allowed to use posts to pen down my workings)

But where do the sulphate ion come in?
Sorry, but I'm kinda lost..

[Borek]

CrO4^2- + 8H+  +3e  ->   Cr^3+  + 4H2O     
2CrO4^2  + 2H+           ->   Cr2O7^2-   + H2O

These?

No! You have to start with CrO₄^3- - that's what is disproportionating, that's the reactant in question, initially present in the solution. Don't stick to the compounds you know - this is a new one.

So more like (unbalanced)

CrO₄^3- + H⁺ -> Cr³⁺ + H₂O

Think how the second half reaction should look alike now.

But where do the sulphate ion come in?
Sorry, but I'm kinda lost..

Sulphate is just a spectator, just like Na⁺. You know what the net ionic reaction is?

[©h£m1§†®¥]

CrO₄^3- + 8H⁺ ---> Cr³⁺ +4H₂O
2CrO₄^3- + 2H⁺ ---> Cr₂O₇^2- + H₂O +2e^-
Is this correct? And is the oxidation state of Cr in Na₃CrO₄ +5?

[Borek]

CrO₄^3- + 8H⁺ ---> Cr³⁺ +4H₂O

No, that's not OK. Charges on both sides of reaction must be identical. You are missing electrons.

And is the oxidation state of Cr in Na₃CrO₄ +5?

Yes. Assuming it exists