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

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#### 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  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.

#### 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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#### 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

#### 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.

regards

Ingo
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.

#### Borek

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

Who? Vidya is not the OP.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info, PZWT_s1

#### 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.