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Calculate the concentration of ions following saturated solutions

Calculate the concentration of ions in the following solutions

(a) [I] in AgI in AgI solution with [Ag+] = 9.6E-9 M Ksp=8.3E-17
(b) [Al3+] in Al(OH)3 solution with [OH-] = 3.8E-9 M Ksp=1.8E-25

I got the first one and it is correct:
AgI-------------->Ag + I
I 9.6E-9 0
C -s +s +s
E -s 9.6E-9+s +s w/9.6E-9+s approx. = to 9.6E-9
8.3E-17 = (9.6E-9)(s)

I did the second one (b) exactly the same way and the program is saying off by order of magnitude, I got 4.7E-25
This is how I sat it up
Al(OH)3----------------->Al3+ + OH-
I 3.8E-9
C +s 3.8E-9+s
E -s s approx. 3.8E-9

What am I doing wrong?

Solution Preview

Well, first thing I noticed was that your Ksp doesn't match what I looked up in a general chemistry text. I believe the Ksp of Al(OH)3 should be ...

Solution Summary

This solution is provided in 130 words. It discusses the Ksp value and solubility product expression, and uses simple description to help understand the problem.