Solubility Problem from Chad's Video

[flin5845]

What is the maximum concentration of fluoride ions that could be present in 0.032M Ba(NO3)2 (Ksp,BaF2 = 3.2x10-8)?
A) 1.0x10-3M
B) 2.0x10-3M
C) 5.0x10-4M
D) 1.0x10-6M

BaF2(s) → Ba2+(aq) + 2F-(aq) Ksp = [Ba2+][F-]^2

3.2x10-8 = (0.032)[F-]^2 and so 3.2×10^−8/0.032=[F-]^2

3.2×10−2/3.2×10−8 =[F−]2

(1.0×10−6)^1/2 =[F−] and so [F−]=1.0×10−3M

I do not understand why we are assuming that [F-]=[x]^2 and not [2x]^2??

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[spoog74]

What is the maximum concentration of fluoride ions that could be present in 0.032M Ba(NO3)2 (Ksp,BaF2 = 3.2x10-8)?
A) 1.0x10-3M
B) 2.0x10-3M
C) 5.0x10-4M
D) 1.0x10-6M

BaF2(s) → Ba2+(aq) + 2F-(aq) Ksp = [Ba2+][F-]^2

3.2x10-8 = (0.032)[F-]^2 and so 3.2×10^−8/0.032=[F-]^2

3.2×10−2/3.2×10−8 =[F−]2

(1.0×10−6)^1/2 =[F−] and so [F−]=1.0×10−3M

I do not understand why we are assuming that [F-]=[x]^2 and not [2x]^2??

this question initially threw me off too, but let me explain.

The question is asking for the CONCENTRATION of flouride ions, NOT the molar solubility. If they asked WHAT WAS THE MOLAR solubility then you would be right by multiplying by 2x^2 BUT, all they want is the CONCENTRATION itself.

Hope that made sense.

For these problems all you have to do is watch out for what they are asking for, if they ask for SOLUBILITY go on with the way chad explained it. If they ask for CONCENTRATION just find X and nothing more.