Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Using Ksp to calculate Molar concentration  (Read 715 times)

0 Members and 1 Guest are viewing this topic.

stefufufu

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 7
Using Ksp to calculate Molar concentration
« on: October 26, 2013, 04:48:22 AM »

Ca3(PO4)2 has a solubility product Ksp = 1.0x10-25. Calculate the molar concentration of calcium ions in solution in equilibrium with solid Ca3(PO4)2.

Ca3(PO4)2 Ksp = 1.0x10-25

Ca3(PO4)2  ::equil:: 3Ca+ + 2PO43-

Ksp = [Ca2+]3[PO43-]2 = 1.0x10-25
     =  [3x]3[2x]2 = 108x5 = 1.0x10-25

[Ca2+] = 3x = 3 x 5√1.0x10^-25/108 = 1.176x10-5 moles/litre.

This is as far as I have managed to use notes to work moles/litre out. I'm not sure which way to work out the Molar concentration of the calcium, because I do not have a volume?  M = n/V.

I feel I'm going completely off track.
Logged

magician4

  • Chemist
  • Full Member
  • *
  • Mole Snacks: +63/-10
  • Offline Offline
  • Posts: 503
Re: Using Ksp to calculate Molar concentration
« Reply #1 on: October 26, 2013, 05:38:15 AM »

in your opinion, is there any difference at all between the " moles/litre " you've calculated for [Ca2+] ( and correctly at that, insofar as the systematic forced on you requires it) and the molar concentration M  ?


regards

Ingo

additional remark I: the systematic forced on you in this calculation leads to utterly, utterly wrong results, mostly because PO43- won't remain what it initially was, but would become protonated next to completely.
to a minor degree, this also is true for Ca2+ , as this becomes hydroxylated

additional remark II : the Ksp given to you is wrong by several orders of magnitude.
the original value in literature is 2,07 * 10-33 moles5/L5 at 25 °C


« Last Edit: October 26, 2013, 06:28:09 AM by magician4 »
Logged
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Vidya

  • Full Member
  • ****
  • Mole Snacks: +25/-45
  • Offline Offline
  • Gender: Female
  • Posts: 420
  • Online Chemistry and Organic Chemistry tutor
    • Chemistry Tutor
Re: Using Ksp to calculate Molar concentration
« Reply #2 on: October 26, 2013, 02:16:39 PM »

x is the molar concentration and is in moles per litre
Ksp is the equilibrium constant and the expression has quantities in molar concentrations

magician4

  • Chemist
  • Full Member
  • *
  • Mole Snacks: +63/-10
  • Offline Offline
  • Posts: 503
Re: Using Ksp to calculate Molar concentration
« Reply #3 on: October 26, 2013, 02:36:12 PM »

Quote
x is the molar concentration and is in moles per litre
yes , it is
and 3 x is exactly the concentration of Ca2+ , resulting of x moles of Ca3(PO4)2

hence:
Quote
[Ca2+] = 3x = 3 x 5√1.0x10^-25/108 = 1.176x10-5 moles/litre.
this already is the concentration you're asked to calculate for:

you're already there

regards

Ingo
Logged
There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.
(Douglas Adams)

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1361/-368
  • Offline Offline
  • Gender: Male
  • Posts: 20825
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Using Ksp to calculate Molar concentration
« Reply #4 on: October 26, 2013, 09:49:56 PM »

you're already there

Who? Vidya is not the OP.
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

stefufufu

  • New Member
  • **
  • Mole Snacks: +0/-0
  • Offline Offline
  • Posts: 7
Re: Using Ksp to calculate Molar concentration
« Reply #5 on: October 27, 2013, 01:09:14 PM »

hence:
Quote
[Ca2+] = 3x = 3 x 5√1.0x10^-25/108 = 1.176x10-5 moles/litre.
this already is the concentration you're asked to calculate for:

you're already there

regards

Ingo

So, I have my answer already? Great!

Thank you all for your replies.
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.048 seconds with 23 queries.