Question

At 25C only .0190 mol of the generic salt AB2 is soluble in 1.00L of water. What is the Ksp of the salt at 25C?
AB2<->A^2+ + 2B^-
I took the .0190 mol and doubled it and multiplied by .019 again and got 6.859x10^-6 and got it wrong what should I do?

Please help Dr. Bob

Answer 1

Did you square the 0.019*2 for B^-?
........AB2 ==> A^2+ + 2B^-
I......solid....0.......0
C.....-0.0190..0.019..0.019*2
E......solid...0.019..0.038

Ksp = (A^2+)(B^-)^2
Ksp = (0.019)(0.038)^2
Ksp = 2.74E-5 for the answer. I do it slightly different because it's faster on the calculator.

Ksp = (A^2+)(B^-)^2
Kso = (x)(2x)^2 = 4x^3
Ksp = 4*(0.019)^3 = 2.74E-5.
The usual student mistake is to double the 0.019 to find B^- but forget it is squared.