June 14, 2021, 11:06:46 PM
Forum Rules: Read This Before Posting


Topic: Problem of the week - 09/04/2012  (Read 6784 times)

0 Members and 1 Guest are viewing this topic.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26563
  • Mole Snacks: +1723/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Problem of the week - 09/04/2012
« on: April 09, 2012, 08:15:51 AM »
100 mL solution of 0.01 M CaCl2 was titrated with 0.015 M sodium sulfate. For what volumes of added titrant solution is precipitate free?

CaSO4 pKsp = 4.6

Edit: changed sulfuric acid to sodium sulfate, it wasn't my intent to make it complicated because of the sulfuric acid dissociation.
« Last Edit: April 11, 2012, 09:17:38 AM by Borek »
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline Sophia7X

  • Full Member
  • ****
  • Posts: 248
  • Mole Snacks: +32/-4
  • Gender: Female
  • I, a universe of atoms, an atom in the universe.
Re: Problem of the week - 09/04/2012
« Reply #1 on: April 10, 2012, 06:21:31 PM »
Net ionic equation: Ca+2 + SO4-2 --> CaSO4

Ksp = 10-4.6

K = Q to initiate precipitation.

Q = Ksp = [Ca+2][SO4-2]
[0.1 L*0.01M/(0.1 L + V)][0.015 M*V/(0.1 L + V)] = 10-4.6
V = 0.37 L

< 0.37 L H2SO4 soln?
Entropy happens.

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7984
  • Mole Snacks: +555/-93
  • Gender: Male
Re: Problem of the week - 09/04/2012
« Reply #2 on: April 11, 2012, 08:29:39 AM »
Concentration of SO42- in 0.015 H2SO4 is about 0.006 M. Assumption 0.015 M H2SO4 is fully dissociated is not correct.
AWK

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26563
  • Mole Snacks: +1723/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Problem of the week - 09/04/2012
« Reply #3 on: April 11, 2012, 09:15:53 AM »
Concentration of SO42- in 0.015 H2SO4 is about 0.006 M. Assumption 0.015 M H2SO4 is fully dissociated is not correct.

Good point. I am slightly modifying the question, otherwise it is getting more complicated than I wanted it to be.

It was obvious sooner or late I am going to miss something.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26563
  • Mole Snacks: +1723/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Problem of the week - 09/04/2012
« Reply #4 on: April 13, 2012, 11:11:09 AM »
< 0.37 L H2SO4 soln?

This is NOT a correct answer.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline DrCMS

  • Chemist
  • Sr. Member
  • *
  • Posts: 1275
  • Mole Snacks: +207/-81
  • Gender: Male
Re: Problem of the week - 09/04/2012
« Reply #5 on: April 13, 2012, 01:44:51 PM »
 Up to 27ml and then after 371ml of the sodium sulphate solution?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26563
  • Mole Snacks: +1723/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Problem of the week - 09/04/2012
« Reply #6 on: April 13, 2012, 01:52:07 PM »
Yes.

Sophia7X - you were quite close. Trick is, when you add more titrant calcium gets diluted - and at some point it is so diluted [Ca2+][SO42-] is again below Ksp. That's the meaning of the second root that you ignored.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Sponsored Links