Ok well here's another one

[drpduck]

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I cannot for the life of me understand this problem! Please help!

The solubility of Fe(OH)3 in an aqueous solution was determined to be 4.5 x 10-10 M. The equilibrium concentration of each ion can be determined from the molar solubility and the balanced dissociation reaction of Fe(OH)3. What is the value of Ksp for Fe(OH)3?

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Fe(OH)3-->Fe3+ + 3OH-


Ksp=[Fe3+ ][OH-]3

**(I understand all up to this point, but below I don't)

[OH-]=3[Fe3+]; [Fe3+ ]= 4.5 x 10-10M

Ksp=[ Fe3+ ](3[Fe3+])3=27[Fe3+ ]4

Ksp=(4.5 x 10-10)[394.5x10-10)]3 = 27 (4.5 x 10-10)4

Ksp=1.1 x 10-36

 

[ScottW3]

Kaplan had some good formulas for this kind of problem. The way you wrote will get you the answer, but its longer and a little more complicated. Kaplan says:

For ions of formula MX,
Ksp= [x]^2 where x = molar concentration

For ions of formula MX2,
Ksp= 4[x]^3

For ions of formula MX3 (which we have in this problem),
Ksp= 27[x]^4

This last one is what you came out with in your post. I hope these help. Plus they are not that bad to memorize.

 

[drpduck]

Wow, yea i just saw those, that is much easier! thanks!