Common Ion and Solubility

[flin5845]

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What is the max concentration of F- Ions that could be present in .032M Ba(NO3)2? (KSP BaF2- 3.2x10^-8)

The Ksp=[Ba][F]^2 But since there is a common Ion the Ba become .032M so to find [F] we have 3.2x10^-8=[.032][x]^2. Then 1x10^-6=[x]^2 and then square root and get [x]= 1x10^-3

Ok but here is my question. If there was not a common Ion present and just say the Ksp was given and we wanted molar solubility the expression would be Ksp= [x][2x]^2= 27x^4.

I do not understand why if there is a common ion why the expression is x^2 and not 2x^2....

 

[rockclock]

What is the max concentration of F- Ions that could be present in .032M Ba(NO3)2? (KSP BaF2- 3.2x10^-8)

The Ksp=[Ba][F]^2 But since there is a common Ion the Ba become .032M so to find [F] we have 3.2x10^-8=[.032][x]^2. Then 1x10^-6=[x]^2 and then square root and get [x]= 1x10^-3

Ok but here is my question. If there was not a common Ion present and just say the Ksp was given and we wanted molar solubility the expression would be Ksp= [x][2x]^2= 27x^4.

I do not understand why if there is a common ion why the expression is x^2 and not 2x^2....

This question always comes up, but basically what it boils down to is that if you're only solving for one quantity, then you can call it whatever you want...it's just the quantity you're solving for, so might as well make it x. The reason you use x and 2x in the molar solubility problem is that you have two different concentrations of each product and you know that the relationship between the two is 2:1 based on the stoichiometic ratio. You could just as easily call them x/2 and x or 18q and 36q if you wanted to...your choice. Just keep track of what the final answer you're supposed to report is.

In the same way, in your first problem, you could just as easily call it 2x as long as you keep track of the fact that your final answer will be whatever 2x is, not just x. Your call. Like I said, what you decide to call unknowns themselves is a matter of choice. It's only the relationships between unknowns that are dictated by the chemistry taking place.

 

[spoog74]

What is the max concentration of F- Ions that could be present in .032M Ba(NO3)2? (KSP BaF2- 3.2x10^-8)

The Ksp=[Ba][F]^2 But since there is a common Ion the Ba become .032M so to find [F] we have 3.2x10^-8=[.032][x]^2. Then 1x10^-6=[x]^2 and then square root and get [x]= 1x10^-3

Ok but here is my question. If there was not a common Ion present and just say the Ksp was given and we wanted molar solubility the expression would be Ksp= [x][2x]^2= 27x^4.

I do not understand why if there is a common ion why the expression is x^2 and not 2x^2....

to put this in easier terms, the question is NOT asking for the molar solubility but is asking for the CONCENTRATION of flouride ions meaning just the concentration. The usual (2x)^2 does not play a role