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Which of the following has highest solubility in water?

A. $ Ca(OH)_2 $

B. $ Fe(OH)_3 $

C. $ Cr(OH)_3 $

D. $ Zn(OH)_2 $

And also please explain how can we predict their solubilities when we are not provided with their solubility products.

I know that calcium hydroxide is sparingly soluble. So it cannot be the right option. I think Iron hydroxide is also soluble to a good extent. I haven't got any idea about option C. I think D may be the right answer.

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    $\begingroup$ Hi SkyDreamer welcome to ChemistryS.E.! Can you show us your efforts and your attempts, we can't do your homework see our homework policy! :-) $\endgroup$ – G M May 14 '14 at 17:30
  • $\begingroup$ I know that calcium hydroxide is sparingly soluble. So it cannot be the right option. I think Iron hydroxide is also soluble to a good extent. This is evident as Iron rusts in presence of water vapours, but it is a slow process. I haven't got any idea about option C. I think D may be the right answer. $\endgroup$ – azaidi May 14 '14 at 17:44
  • $\begingroup$ Please help! I have got my final exam tomorrow. $\endgroup$ – azaidi May 14 '14 at 17:47
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    $\begingroup$ What are your thoughts on what the answer is? We can't solve problems for you; we only guide you through them. In addition, iron hydroxide is a gel-like substance and is hydrated. I'm not aware of how this gel-like precipitate may rust. $\endgroup$ – Dissenter May 14 '14 at 17:57
  • $\begingroup$ Ohh. I think I was wrong about Iron hydroxide. I just have got a rough idea that the answer is D. $\endgroup$ – azaidi May 14 '14 at 18:04
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You can't predict solubility reasoning, you have to remember solubility rules in your case this one:

  1. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and $\ce{Al^3+}$ are insoluble. Thus, $\ce{Fe(OH)3, Al(OH)3, Co(OH)2}$ are not soluble.

So in this case $\ce{Ca(OH)2}$ even if slightly soluble theoretically should be the right answer.

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