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kzteam5226
Joined: 02 Jul 2009
Posts: 9
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 Posted: Thu Jul 09, 2009 1:39 pm Post subject: Question 33 |
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| Why does 2F- = (2X)^2? I understand that [F-][F-] = X^2... but why is it 2X^2? |
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michaelgar7090
Joined: 11 Mar 2009
Posts: 11
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| Posted: Thu Jul 09, 2009 6:08 pm Post subject: |
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| For every mole of Mg formed, 2 moles of F- are formed. So if we say concentration of Mg is s, then concentration of F is 2s. Since we're seeking solubility, we use Ksp =s(2s)^2 = 4s^3 . |
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kzteam5226
Joined: 02 Jul 2009
Posts: 9
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| Posted: Fri Jul 10, 2009 2:52 am Post subject: |
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| so the fact that it is 2s makes sense.. however by squaring it.. aren't we accounting for it's doubled concentration twice? should it just be 2s and not squaring 2s because we already account for the fact that there is twice as much F- as there is Mg 2+? |
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jellywing_2058
Joined: 04 May 2009
Posts: 179
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| Posted: Mon Jul 12, 2010 3:15 pm Post subject: |
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dnpgr16513
Joined: 31 Dec 1969
Posts: 46Post
Posted: 11 Jul 2010 12:20 pm Post subject:
| Quote: | How do you take the cuberoot of any number without a calculator?! Taking the square root is a little easier but this is a little more tough to estimate......
thanks. |
I agree with you, but here is how I approached the problem:
As shown in the explanation:
MgF2(s) --> Mg2+(aq) 2F-(aq)
Ksp = [Mg2+][F-]2 = 6.5 x 10-9
For convenience, let us denote [Mg2+] as X.
Thus:
6.5 x 10^(-9) = X(2X)^2 = X(4X^2) = 4X^3
X = cube root of {(6.5 x 10(-9))/4}
Now I thought, 6.5 x 10(-9) is close to 6.4 x 10(-9) and 6.4x 10(-9)/4 = 1.6 x 10(-9)
so X = ~(1.6 x 10-9)^1/3
Then I extracted the exponent right away :(10^-9)^(1/3)= 10^-3
Leaving:
X = ~[(1.6)^1/3] x (10^-3)
Already that eliminates answer choices B and C since these have different exponent values.
Then I knew that
2 > [1.6^(1/3)] > 1
--> because anything <1 would give a smaller number than its original value (For example, 0.2^3 = 0.008)
--> and <2 because 2^3 = 8 and out of answer choices A and D there is no such value.
(Remember that X is actually 1/2 [F-], so without looking at the exponents (since it has already been isolated and dealt with), 1/2 A is 1.2 and 1/2 B is 0.6).
This means the answer must be A! |
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jsfkt78927
Joined: 06 Jul 2010
Posts: 44
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| Posted: Tue Jul 27, 2010 3:20 pm Post subject: |
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Also another what to look at the problem,...
If you get to (1.6 ^1/3) x 10^-3 and youre trying to estimate the cube root, try this:
It might help to know ur squares up to twenty, although some instructors only recommend 15.
13^2 = 169
14^2 = 196
15^2 = 225
16^2 = 256
17^2 = 289
18^2 = 324
19^2 = 361
20^2 = 400
maybe anything past 15 is a little too much but hey... what eva.
Then you have 1.6 which is close to 1.69, and the square of that would be 1.3.... right?
so if youre still with me,
the cube root should be less than the square root which is close to 1.2
Now if youre like me and did that and forgot to multiply by two, well then.... better luck next time. |
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