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Topic: Given Ksp of a saturated solution, how do I calculate [F-]?  (Read 3139 times)

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Offline Rhydos

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Given Ksp of a saturated solution, how do I calculate [F-]?
« on: June 13, 2010, 12:41:41 PM »
Calculate [F-] in a saturated solution of MgF2 whose Ksp = 6.4x10-9

In what steps do I proceed to answer this question?
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Offline Jorriss

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Re: Given Ksp of a saturated solution, how do I calculate [F-]?
« Reply #1 on: June 13, 2010, 01:46:36 PM »
You can do this if you assume there is no other source of magnesium or fluoride contributing to the system.

Have you written down your Ksp expression yet? And if so, is there anything you know that relates the amount of magnesium to the amount of fluoride?
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Offline AWK

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Re: Given Ksp of a saturated solution, how do I calculate [F-]?
« Reply #2 on: June 14, 2010, 02:02:19 AM »
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Offline opti384

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Re: Given Ksp of a saturated solution, how do I calculate [F-]?
« Reply #3 on: June 14, 2010, 07:57:00 AM »
1. Write the Ksp expression of MgF2.

[Mg2+][F-]2 = 6.4 x 10-9

2. Since MgF2  :rarrow: Mg2+ + 2F-,

[Mg2+] will be two times more than [F-]

3. We assume [F-] as y

Therefore 2y x y2 = 6.4 x 10-9

Of course as Jorriss said, this is possible when we assume that there is no other source of magnesium or fluoride.
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