Solubility

[anteater85]

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Hi guys,
I'm very confused about the solution that KBB has for this question:

The solubility of Fe(OH)3 in an aqueous solution was determined to be 4.5 x 10 ^-10 mol/L. what is the value of the Ksp for the Fe(OH)3?

solution:

Fe(OH)3-->Fe + 3 OH
Ksp= [Fe][Oh]^3
[OH]= 3[Fe]

I don't get this part? what the OH is equal to 3 Fe?

 

[joonkimdds]

Fe(OH)3 dissociates into 1 Fe and 3 OH.

thus when we have 1 mol of Fe, we have 3 mol of OH.

That's why [OH] = 3[Fe]

we usually write make [ ] into x = solubility

Fe(OH)3 = [x][3x]^3 = 27x^4 = 27(4.5 x10^-10)^3 and u can continue~ 🙂