chem question ..help please =/

[jkb53433]

i think this might be such a easy answer but i cant understand it

question is
CaF2 is added to a .1 M Ca(NO3)2 solution. at what concentration of F- will CaF2 begin to precipitate? (Note: Ksp of CaF2 is 4x10^-11)
a. 4x10^-5
b. 2x10^-5
c. 4x10^-10
d. 2x10^-10


answer is b.

help

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[Kahr]

Okay, first thing I do is set up the Ksp expression since we're given a Ksp.

Ksp = [Ca] [F]^2 (Notice, CaF2 <-> Ca + 2F)

Now, they want to know at what concentration [F] will cause precipitate to form. We know that at Ksp, we're in equilibrium, so the reaction isn't going to be forming precipitate. So we need to figure out what [F] is at equilibrium, and see in our answers which one is immediately greater (le chatelier), because once a greater amount of [F] occurs, it will push the reaction to the left and form precipitate.

4*10^-11 = [0.1] [2x]^2
(Notice, [Ca] has a common ion effect from the Ca(NO3)2, so first it would be [x + 0.1] but since x is very small, we just use 0.1)

4*10^-10 = 4x^2 (I'm sure you can do the math)

1*10^-10 = x^2

1*10^-5 = x
Okay, so we now know that at equilibrium (Ksp), concentration of [F] is this number, so we see in our answers that (b) is the number most closely larger, so that is when precipitate will BEGIN to form (shift to the left, forming CaF2). Notice that choice (a) is also larger, and would also form precipitate (just that it would begin at the smaller number).

Hope this helps.

Edit: Btw, I don't think it's such an easy problem, they're asking for a strange thing (not just a Ksp, or concentration, etc), they threw in a common ion as well, and the balanced reaction uses a non 1:1 molar ratio.

 

[wizi]

Okay, first thing I do is set up the Ksp expression since we're given a Ksp.

Ksp = [Ca] [F]^2 (Notice, CaF2 <-> Ca + 2F)

Now, they want to know at what concentration [F] will cause precipitate to form. We know that at Ksp, we're in equilibrium, so the reaction isn't going to be forming precipitate. So we need to figure out what [F] is at equilibrium, and see in our answers which one is immediately greater (le chatelier), because once a greater amount of [F] occurs, it will push the reaction to the left and form precipitate.

4*10^-11 = [0.1] [2x]^2
(Notice, [Ca] has a common ion effect from the Ca(NO3)2, so first it would be [x + 0.1] but since x is very small, we just use 0.1)

4*10^-10 = 4x^2 (I'm sure you can do the math)

1*10^-10 = x^2

1*10^-5 = x
Okay, so we now know that at equilibrium (Ksp), concentration of [F] is this number, so we see in our answers that (b) is the number most closely larger, so that is when precipitate will BEGIN to form (shift to the left, forming CaF2). Notice that choice (a) is also larger, and would also form precipitate (just that it would begin at the smaller number).

Hope this helps.

Edit: Btw, I don't think it's such an easy problem, they're asking for a strange thing (not just a Ksp, or concentration, etc), they threw in a common ion as well, and the balanced reaction uses a non 1:1 molar ratio.

Kahr, your math is perfect but Ill add something to it.

First, Concentration of F is not X. Its 2x, therefore the answer is exactly 2*10^-5
CaF2 = Ca + 2F
x x 2x

Second, all of the answer choices have greater concentration than Ksp. Not just a and b.

 

[Kahr]

Kahr, your math is perfect but Ill add something to it.

First, Concentration of F is not X. Its 2x, therefore the answer is exactly 2*10^-5
CaF2 = Ca + 2F
x x 2x

Second, all of the answer choices have greater concentration than Ksp. Not just a and b.

Ahh yes, sorry, I forgot to re-apply that after deriving x.

We aren't comparing the answer choices to the Ksp value, we are comparing them to the concentration of "F", therefore choices c and d are automatically out because they're smaller than the concentration of "F", whereas "a" is larger, and "b" is the same. At "a" we'll definitely have precipitate, and I'm guessing at equilibrium, we will see some precipitate, at least if the answer is "b". Care to confirm this?

 

[jkb53433]

Ah I c...yes u guys r right ...thank you

 

[qkchen]

Kahr, your math is perfect but Ill add something to it.

First, Concentration of F is not X. Its 2x, therefore the answer is exactly 2*10^-5
CaF2 = Ca + 2F
x x 2x

Second, all of the answer choices have greater concentration than Ksp. Not just a and b.

NEVERMIND: the answer is 2x, that's why.