QUESTION ABOUT Ksp

[cathyrn]

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I am getting really confused by Ksp questions:

For instance if BaF2 dissociates the Ksp will be (Ba)(F)2. Why won't it be
(Ba)(2F)2 ?

 

[rocketbooster]

I am getting really confused by Ksp questions:

For instance if BaF2 dissociates the Ksp will be (Ba)(F)2. Why won't it be
(Ba)(2F)2 ?

it is...
Ksp=[Ba][F]^2
[F]=2x
[Ba]=x
Ksp=(x)[(2x)^2]
Ksp=4x^3

Ksp=solubility product
x=molar solubility
if the cation to anion ratio is 1:1, you can use the Ksp to determine how soluble something is. if the cation to anion ratio is not 1:1, you need to use the molar solubility. using the molar solubility should be used for both cases, but you can get-by by using just the Ksp if the cation:anion is 1:1. so on an MCAT question, if it tells you to give the sequence of the following examples of increasing solubility and all of the examples have that 1:1 ratio, you can base your answer on the Ksp values (lower value=less soluble=precipitates first)

 

[Bernoull]

it is...
Ksp=[Ba][F]^2
[F]=2x
[Ba]=x
Ksp=(x)[(2x)^2]
Ksp=4x^3

rocketbooster is absolutely right. Ksp is simply a statement of the law of mass action:

[C]c [D]d
------------- = Keq
[A]a b

so we ignore pure solids so we only have the numerator left. Ksp=[Ba][F]^2

We can also express Ksp in terms of molar solubility which is what rocketbooster did. Ksp=4x^3 where x is the concentration at saturation which is usually what u need to calculate.

 

[cathyrn]

okay question states that BaF2 Ksp= 1.7 x10^-6 and concentration of Ba in a saturated soln is 4.3 x 10^-3. what is the concentration of fluoride ions in soln?

I did it wrong by calculating 1.7 x 10^-6= (4.3 x 10^-3) (2x)^2

the answer was 2.0 x 10^-2 please explain

 

[Bernoull]

okay question states that BaF2 Ksp= 1.7 x10^-6 and concentration of Ba in a saturated soln is 4.3 x 10^-3. what is the concentration of fluoride ions in soln?

I did it wrong by calculating 1.7 x 10^-6= (4.3 x 10^-3) (2x)^2

the answer was 2.0 x 10^-2 please explain

Okay FYI, even if molar solubility wasn't provided which usually is the case, this is how u find it:
Ksp = [Ba][F]^2 =4x^3 = 1.7E-6
therefore x^3=0.425E-6 and x= 4.25E-3 (molarity) that's the value given.

Now to answer you, we want [F],
given Ksp = [Ba][F]^2, [F] =sqrt(Ksp/[Ba])

[F]^2 = 1.7E-6/4.25E-3 =4E-4
[F] =2E-2 M

 

[cathyrn]

Thanks!!!

 

[Bernoull]

Thanks!!!

anytime..

 

[Cjc555777]

just to help clarify, the reason you use 2x in those equations is because x is the concentration of of barium. You need twice as much fluorine than barium that's why you put in there 2x. but when they give you the amount of barium you don't need it, and the value turns into x because in a problem like that the barium concentration would be higher or lower and you just solve. it's a little confusing but just remember what x actually is not just putting x in when you see one of these problems. hope that helps