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Offline Spectrums

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Ksp Calculation Question
« on: November 20, 2016, 02:32:37 PM »
For a saturated solution of an unknown metal hydroxide, M(OH)2, 12.75 mL of this solution were required to neutralize 25.00 mL of 0.02 M HCl. Calculate the KSP of the metal hydroxide.

I've tried the problem two different ways because I'm not sure whether I'm supposed to divide the concentration of M(OH)2 by 2 to get the concentration of OH- or if I should multiply it by 2.

M(OH)2  ::equil:: M2+ + 2OH-
This makes mole ratio 1 : 2.

Using C1V1=C2V2, the concentration of M(OH)2 is:
C1(12.75)=(0.02)(25.00)
= 0.039 M

From this I assumed that the concentration of M2+ was also 0.039 given the 1:1 mole ratio and the concentration of OH- is 2 times the concentration of M(OH)2 making it 0.078.

This makes the KSP=[M2+][OH-]2
= [0.039][0.078]2
= 2.373 x 10-4

If the concentration of M(OH)2 should be divided by 2 to get the concentration of OH-, this makes the KSP = [0.039][0.0195]2
= 1.483 x 10-5

If neither of these are correct, could someone please point me in the right direction?

Offline AWK

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Re: Ksp Calculation Question
« Reply #1 on: November 20, 2016, 02:52:24 PM »
Quote
From this I assumed that the concentration of M2+ was also 0.039 given the 1:1 mole ratio and the concentration of OH- is 2 times the concentration of M(OH)2 making it 0.078.
Wrong - M(OH)2 !!!
You titrated OH-
AWK

Offline Spectrums

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Re: Ksp Calculation Question
« Reply #2 on: November 20, 2016, 03:39:29 PM »
Quote
From this I assumed that the concentration of M2+ was also 0.039 given the 1:1 mole ratio and the concentration of OH- is 2 times the concentration of M(OH)2 making it 0.078.
Wrong - M(OH)2 !!!
You titrated OH-

If the assumption regarding the concentration of M2+ is correct in it being equal to the concentration of M(OH)2 but the assumption regarding the concentration of OH- is wrong, wouldn't this make the second solution I posted correct? If not, can you give me some concrete advice on how to approach the problem?

Offline AWK

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Re: Ksp Calculation Question
« Reply #3 on: November 20, 2016, 03:58:05 PM »
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the KSP = [0.039][0.0195]2
Also wrong - concetration of Me2+ is 0.0195 and you put is as concentration of OH-
AWK

Offline Spectrums

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Re: Ksp Calculation Question
« Reply #4 on: November 20, 2016, 05:15:18 PM »
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the KSP = [0.039][0.0195]2
Also wrong - concetration of Me2+ is 0.0195 and you put is as concentration of OH-

So [OH-] = [M(OH)2] because OH- is titrated & [M2+] = (0.5)[OH-] because of 1:2 mole ratio?

Offline AWK

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Re: Ksp Calculation Question
« Reply #5 on: November 20, 2016, 05:32:28 PM »
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So [OH-] = [M(OH)2]
False statement
So = 0.5[OH-] = [M(OH)2]

Titration of base with HCl is in fact
H+ + OH- = H2O
Since M(OH)2 contains 2 hydroxyl group in stoichiometric unit of base then
concentration of M2+ = 0.5 times concentration of OH-
AWK

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