The hydroxide ion has the formula OH. The solubility-product constants for three

Question

The hydroxide ion has the formula OH. The solubility-product constants for three generic hydroxides are given here.

Part A:

The removal of an ion is sometimes considered to be complete when its concentration drops to 1.00×106  M. What concentration of hydroxide would cause Y2+ to "completely" precipitate from a solution? Give Units

Part B:

At a pH of 10.5, arrange the solutions containing the following generic hydroxides in order of decreasing concentration of the cation remaining in the solution (i.e., in order of increasing completeness of precipitation).

Rank from highest concentration to lowest

Explanation / Answer

Part A:
Ksp = [Y2+][OH-]2
[Y2+] = 1*10-6 M
2.5*10-10 = 1*10-6 * [OH-]2  
[OH-] = 0.015811 M

Part B:
PH + POH = 14
10.5 + POH =14
POH = 14-10.5 = 3.5
3.5 = POH = -log10 [OH-]
10-3.5 = 3.16227*10-4 M = [OH-]
Now using the above value for [OH-] , calculate X,Y and Z
i) Ksp = [X] [OH-]
   2.40*10-8 = [X] * 3.16227*10-4   
   7.5894*10-5 M = [X]
ii) 2.50*10-10 = [Y2+] (3.16227*10-4 )2
   [Y2+] = 2.5*10-3 M
iii) 6.30*10-15 = [Z3+] (3.16227*10-4 )3
      [Z3+] = 1.9922*10-4 M
decreasing order of the cations concentration ,  [Y2+] >  [Z3+] >  [X]
     

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