In: Chemistry
If aluminum hydroxide is added to an acidic solution with a pH of 1.93, what is the new pH after equilibrium is established? This problem is a bit tricky. So, extra help is given. The following ICE Table is useful: ...
Al(OH)3(s) ↔ Al3+(aq) + 3OH-(aq) Ksp = 2.0 x10-32 M4
I: ...Excess ... ...0 ......... ...from pH
C:.....-s....... ......s ...... ........3s
E:...Excess ... ...s.......... .... ? (can't add)
Hint, hydroxide produced neutralizes acid. [H+] =[H+]o -3s for 3s < [H+]o. From [H+] hydroxide concentration may be obtained. Math is simple, but you need to notice what reasonable approximation is useful.
Ksp of Al(OH)3 = 2.0 x 10^-32
Ksp = [Al3+][OH-]^3
pH = 1.93
pOH = 14 - pH = 12.07
pOH = -log[OH-]
[OH-] = 8.5 x 10^-13 M
Al(OH)3(s) <===> Al3+(aq) + 3OH-(aq)
I ex - 8.5 x 10^-13
C -x +x +3x
E ex x 8.5 x 10^-13 + 3x
So,
2 x 10^-32 = (x)(8.5 x 10^-13 + 3x)^3
OH- concentration is very small
2 x 10^-32 = (x)(3x)^3
2 x 10^-32 = 27x^4
x = [OH-] = 5.22 x 10^-9 M
pOH = -log[OH-] = 8.28
pH = 14 - pOH = 5.72