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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: apathy on November 12, 2009, 12:28:27 AM

Title: Solubility of copper(II) sulfate with pH of solution
Post by: apathy on November 12, 2009, 12:28:27 AM

Hiya!

I'm trying to calculate how much of copper (II) sulfate can be dissolved in solution with different pH. As far as I understand, if we decrease the pH of solution, more copper (II) sulfate can be dissolved. Following is my approach to this problem (which might be completely wrong, because I got really confused and starting to doubt my understanding of this whole equilibrium and solubility product concepts):

The reactions involved are:
CuSO₄  ::equil:: Cu²⁺ + SO₄^2-
Cu²⁺ + 2 OH^- ::equil:: Cu(OH)₂

Because of the second reaction, I thought that by adding more OH^- (increasing pH), then copper (II) hydroxide would precipitate, hence limiting the "solubility" of CuSO₄.

The solubility product constant of copper (II) hydroxide according to Knovel Critical Table is 2.20 x 10^-20.
K_sp = [Cu²⁺] [OH^-]²

For solution with pH = 5, I calculated the concentration of OH^- as:
pOH = -log ([OH^-]) = 14 - pH
[OH^-] = 10^-9

From K_sp and [OH^-],
[Cu²⁺] = 2.20 x 10^-2

Does this mean only 2.2 x 10^-2 mol of CuSO₄ can be dissolved in 1L of water if the pH is maintained at 5? Which would translate to approximately 3.51 g/L of anhydrous CuSO₄. The solubility of CuSO₄ given in the MSDS that I checked was 243 g/L at 0°C.

As all of you can probably tell at this point, I am completely clueless as to what I'm doing  ???, so any helps would be greatly appreciated! Thanks in advance  :)

Title: Re: Solubility of copper(II) sulfate with pH of solution
Post by: AWK on November 12, 2009, 01:51:24 AM

You discuss two different problems - solubility of copper sulfate which is temperature dependent (common ion - sulfate and ionic strenght also change solubility of this compound) and solubility of Cu(OH)₂which starts to precipitate in sligthtly basic solution.

Title: Re: Solubility of copper(II) sulfate with pH of solution
Post by: apathy on November 12, 2009, 06:29:47 AM

Thanks for the reply.

Does it mean I am not supposed to relate them at all? Sorry, I guess I should've stated my objective first. I am trying to dissolve copper (II) sulfate in acidic or basic solution without getting copper (II) hydroxide to precipitate. My prof suggested that I should add sulfuric acid or sodium hydroxide to alter the pH of the solution.

I chose a pH of 5 in the calculation, because I've dissolved over 10g/L of CuSO₄ without getting any precipitation at pH close to 5




Edit: added additional infos