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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)
s=8.6E-9=[I]

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 This solution is FREE courtesy of BrainMass!

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 1.8E-33, not 1.8E-25.

The next thing thing you need to consider is the actual solubility product expression.
For example, in the first one, the expression is:

Ksp = [Ag+][I-]

as you know.

However, for the second one it is:

Ksp = [Al3+][OH-]^3

not

Ksp = [Al3+][OH-]

Why? Because the equation is this:

Al(OH)3 <-----> Al3+ + 3 OH-

If you consider the correct Ksp expression, you should be fine.

Hopefully, that gets you back on track!