Help please with this precipitation exercise

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Discussion Overview

The discussion revolves around calculating the minimum pH required to completely dissolve 10 mg of Fe(OH)3 in an acidic solution, given the solubility product constant (Ks) for Fe(OH)3. Participants explore the equilibrium conditions and the necessary concentrations of hydronium and hydroxide ions in the solution.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant calculates the concentration of iron ions and hydroxide ions, proposing that the concentration of hydronium ions must neutralize the hydroxide ions to shift the equilibrium.
  • Another participant questions the clarity of the initial approach, asking for clarification on the "difference" mentioned and the implications of water autoionization.
  • A later reply suggests that the problem may require finding the final stable pH to prevent precipitate formation, noting that assumptions about the acid's strength are necessary for a complete solution.
  • Concerns are raised about the ambiguity of the problem statement and the potential for buffered solutions affecting pH during dissolution.

Areas of Agreement / Disagreement

Participants express differing interpretations of the problem's requirements and the assumptions needed to solve it. There is no consensus on the correct approach or the necessary conditions for the calculations.

Contextual Notes

The discussion highlights limitations related to assumptions about the strength of the acid and the buffering capacity of the solution, which are not explicitly addressed in the problem statement.

BeeG
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Knowing Ks=6⋅10−38 for Fe(OH)3 in neutral solutions, calculate the minimum pH of an acidic solution in order to completely dissolve 10 mg of Fe(OH)3. Data: Volume = 0.1 L.​

I set out the equation for the equilibrium constant. having first calculated the concentration or iron ions (9.35899*10-4 M), and left the hydroxide concentration as an unknown:
Ks = 9.35899*10-4 M * 27 C3
Solving for C, I get 1.334*10-12 M, and so my guess is that the concentration of the hydronium ions must be the difference, as it has to neutralise the hydroxide ions in order to move the equilibrium towards the left. Doing this, I get 9.35899*10-4 M concentration for hydronium ions, which corresponds to pH = 3.03.
Is this approach and solution correct?


 
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BeeG said:
my guess is that the concentration of the hydronium ions must be the difference

Difference between what and what?

Do you know what the water autoionization is? You have calculated concentration of OH-. What is pH of this solution?
 
Borek said:
Difference between what and what?

Do you know what the water autoionization is? You have calculated concentration of OH-. What is pH of this solution?

I do know what water autoionization is. I meant the difference between the initial hydroxide concentration, 9.35899*10-4 M, and the needed concentration for Q = Ks, which is what I calculated as 'C'.

What the problem wants me to do, I think, is calculate the concentration of hydronium ions in an acid solution that I have to add to the one I already have in order to dissolve the precipitate.

Please help. I am at complete loss at what to do and cannot seem to find any guidance in my textbook.
 
Last edited:
Is this the exact statement of the problem?
 
Question is a bit ambiguous.

My bet is that you are asked to find the final pH at which the solution will be stable (that is, precipitate will not form). Otherwise there is no way to find the answer without making assumptions about the strength of the acid present. Not to mention the fact pH can be buffered (in which case it doesn't change during the dissolution).

I gave you hints about how to proceed with such a case in my previous post.

Once you have that and assuming solution contains a strong (fully dissociated) acid it is trivial to calculate (just from the stoichiometry of the dissolution) by how much the initial pH should be lower - but as I said above, it requires an assumption that the acid is strong, and the question doesn't justify this assumption.
 

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