General Chem. Question (Ksp and solubility)

[UnimaasMED]

Hi guys,

I have a question regarding the example question from KBB.
The sample question is: 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 Fe(OH)3?

Now this is what I don't get from the explanation in the book. Why is [OH^-]=3[Fe^3+] 😕

Thanks!

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[sfoksn]

Because when you dissolve Fe(OH)3, you get 3 OH's and a Fe.

OH is 3 times as much as Fe. Say the solubility was 1, for simplicity.

so [OH] would be 3, [Fe] would be 1.

3 = 3(1)

 

[lala05]

Hi guys,

I have a question regarding the example question from KBB.
The sample question is: 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 Fe(OH)3?

Now this is what I don't get from the explanation in the book. Why is [OH^-]=3[Fe^3+] 😕

Thanks!

The balanced equation for the reaction is:

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

Notice you have 3 moles of OH^- for every 1 mole of Fe^3+.

Therefore, the concentration of OH will be 3 times the concentration of Fe^3+

Ksp = [Fe^3+][OH-]^3
let x denote [Fe^3+] --> Ksp = (x)(3x)^3 = 27x^4
You can plug in the solubility (4.5 x 10^-10) for x and you can solve for Ksp

 

[herkulease]

here you have a ratio of 1 Fe3+ to 3 OH-

so ksp = [Fe3+][3*OH-]^3

plug the molar solubility they provided and solve.

 

[RCT PC CRN]

Solubility x=4.5 * 10^-10

Ksp = x * (3x)^3 = 27x^4 = 27 * (4.5 * 10^-10)^4

(I can't be bother to solve further...Im laaazziiiiiiiiii 🙂 )

For every mole of Fe ion you would get 3 mole of OH ion Fe=3 OH ...I don't understand why would they put other way around !!! 😕

 

[RCT PC CRN]

Wow...just between my typing and Click submit reply ...there are already 4 responses...go SDNers...very active !!!

 

[UnimaasMED]

That makes perfect sense now😀. Thanks to everybody for responding.