May 06, 2021, 12:31:34 PM
Forum Rules: Read This Before Posting


Topic: Why is Silver Chromate soluble in Nitric Acid but not water?  (Read 18197 times)

0 Members and 1 Guest are viewing this topic.

Offline stephen6119

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Why is Silver Chromate soluble in Nitric Acid but not water?
« on: December 02, 2007, 05:46:17 PM »
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

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26485
  • Mole Snacks: +1721/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #1 on: December 02, 2007, 05:59:00 PM »
H2CrO4 is strong, or weak acid?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline stephen6119

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #2 on: December 02, 2007, 06:04:43 PM »
H2CrO4 is strong, or weak acid?

H2CrO4 is a weak acid.

Offline Padfoot

  • Full Member
  • ****
  • Posts: 221
  • Mole Snacks: +23/-2
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #3 on: December 02, 2007, 06:47:09 PM »
In solution, there is an equilibruim between chromate and dichromate:

2CrO42- + 2H+ <---> Cr2O72- + H2O

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26485
  • Mole Snacks: +1721/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #4 on: December 02, 2007, 07:07:13 PM »
H2CrO4 is strong, or weak acid?

H2CrO4 is a weak acid.

What happens to weak acids in acidic solutions?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline Padfoot

  • Full Member
  • ****
  • Posts: 221
  • Mole Snacks: +23/-2
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #5 on: December 04, 2007, 08:40:59 PM »
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  :)

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26485
  • Mole Snacks: +1721/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #6 on: December 05, 2007, 03:24:27 AM »
In low pH chromate is protonated, pushing dissolution equilibrium to the right. Nothing fancy, you are just removing one of the products (CrO42-) from the solution :)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7984
  • Mole Snacks: +555/-93
  • Gender: Male
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #7 on: December 05, 2007, 06:47:00 AM »
H2CrO4 is not so weak pKa ~ -1. But the driving force for solubility is of course the removing of CrO42- anoions by the equilibrium 2CrO42- + 2H3O+ <=> Cr2O72- + H2O with huge K
AWK

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26485
  • Mole Snacks: +1721/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #8 on: December 05, 2007, 07:25:11 AM »
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.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7984
  • Mole Snacks: +555/-93
  • Gender: Male
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #9 on: December 05, 2007, 08:28:17 AM »
Quote
At pH 4 HCrO4- dominates solution.
Well, silver chromate is sligtly soluble in acetic acid.
AWK

Offline Padfoot

  • Full Member
  • ****
  • Posts: 221
  • Mole Snacks: +23/-2
Re: Why is Silver Chromate soluble in Nitric Acid but not water?
« Reply #10 on: December 05, 2007, 05:19:50 PM »
In low pH chromate is protonated, pushing dissolution equilibrium to the right. Nothing fancy, you are just removing one of the products (CrO42-) 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!

2CrO42- + 2H3O+ <=> Cr2O72- + H2O
AWK you should have quoted me!  ;)

Sponsored Links