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Topic: Using Ksp to calculate Molar concentration  (Read 13921 times)

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Offline stefufufu

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Using Ksp to calculate Molar concentration
« on: October 26, 2013, 10:48:22 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.

Offline magician4

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Re: Using Ksp to calculate Molar concentration
« Reply #1 on: October 26, 2013, 11:38:15 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, 12:28:09 PM by magician4 »
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Offline Vidya

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Re: Using Ksp to calculate Molar concentration
« Reply #2 on: October 26, 2013, 08:16:39 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

Offline magician4

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Re: Using Ksp to calculate Molar concentration
« Reply #3 on: October 26, 2013, 08:36:12 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
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Offline Borek

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Re: Using Ksp to calculate Molar concentration
« Reply #4 on: October 27, 2013, 03:49:56 AM »
you're already there

Who? Vidya is not the OP.
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Offline stefufufu

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Re: Using Ksp to calculate Molar concentration
« Reply #5 on: October 27, 2013, 07:09:14 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.

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