[stephen6119]

This is a question in my lab that I am unsure about...

Although Ag₂CrO₄ is insoluble in water, it is soluble in dilute HNO₃. Explain, using chemical equations...

Can anyone help me out?

Thanks,
Stephen

[Borek]

H₂CrO₄ is strong, or weak acid?

[stephen6119]

H₂CrO₄ is strong, or weak acid?

H₂CrO₄ is a weak acid.

[Padfoot]

In solution, there is an equilibruim between chromate and dichromate:

2CrO₄^2- + 2H⁺ <---> Cr₂O₇^2- + H2O

[Borek]

H₂CrO₄ is strong, or weak acid?

H₂CrO₄ is a weak acid.

What happens to weak acids in acidic solutions?

[Padfoot]

Since the original poster isn't coming back,  I suppose I'll have to post.   I was wondering how H2CrO4 came into it.  Can you (or someone else) explain it.  My initial thought was that the equilibruim I wrote would be pushed to the right and this would cause more to dissolve.  Is there a better reason?

Thanks 

[Borek]

In low pH chromate is protonated, pushing dissolution equilibrium to the right. Nothing fancy, you are just removing one of the products (CrO₄^2-) from the solution

[AWK]

H2CrO4 is not so weak pKa ~ -1. But the driving force for solubility is of course the removing of CrO₄^2- anoions by the equilibrium 2CrO₄^2- + 2H₃O⁺ <=> Cr₂O₇^2- + H₂O with huge K

[Borek]

H2CrO4 is not so weak pKa ~ -1

First dissociation step is of no use here.

My source tells pKa1 = 0.74 and pKa2 = 6.49. At pH 4 HCrO₄^- dominates solution. Reality is more complex, as this protonation is a first step to dichromate synthesis, but pKa2 alone is enough to explain silver chromate solubility in diluted acid.

[AWK]

At pH 4 HCrO4- dominates solution.

Well, silver chromate is sligtly soluble in acetic acid.

[Padfoot]

In low pH chromate is protonated, pushing dissolution equilibrium to the right. Nothing fancy, you are just removing one of the products (CrO₄^2-) from the solution

oh OK.  Thanks Borek for your clarification 
From AWK's post I guess both are pretty good factors though, so we were all right!

2CrO₄^2- + 2H₃O⁺ <=> Cr₂O₇^2- + H₂O

AWK you should have quoted me!