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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: darkprince304 on March 21, 2012, 01:26:27 AM

Title: Solubility Product Question
Post by: darkprince304 on March 21, 2012, 01:26:27 AM: Hi Im new here

Why is Mg(OH)2 sparingly soluble in water but highly soluble in ammonium
chloride solution.
Can I know the reason?

Thanks :)
Title: Re: Solubility Product Question
Post by: UG on March 21, 2012, 02:01:11 AM: Because NH₄⁺ is an acid and Mg(OH)₂ is a base, the two can react. Does that answer your question?
Title: Re: Solubility Product Question
Post by: darkprince304 on March 21, 2012, 02:07:58 AM: Um, thanks but I still don't get it. If they both can react how is it soluble in NH4Cl and what is the relation of solubility product with it?
Title: Re: Solubility Product Question
Post by: UG on March 21, 2012, 02:13:35 AM: Quote from: darkprince304 on March 21, 2012, 02:07:58 AM
If they both can react how is it soluble in NH4Cl and what is the relation of solubility product with it?
You can think of the dissolution as an equilibrium Mg(OH)₂ (s) ::equil:: Mg²⁺ + 2OH^-
Now if the hydroxide ions react with the NH₄⁺ what will happen to the equilibrium?
Title: Re: Solubility Product Question
Post by: darkprince304 on March 21, 2012, 02:19:36 AM: It will go backward?
Title: Re: Solubility Product Question
Post by: UG on March 21, 2012, 02:27:04 AM: Quote from: darkprince304 on March 21, 2012, 02:19:36 AM
It will go backward?
Eh, not quite. The reaction with NH₄⁺ reduces the amount of OH^- ions, ie, they are 'consumed' from the equilibrium with magnesium hydroxide solid. So essentially, you are removing OH^- ions from the expression Mg(OH)₂ (s) ::equil:: Mg²⁺ + 2OH^-, this will cause the solid to dissolve more in order to produce more OH^- ions to react with NH₄⁺. The result of this is that the solid has become more soluble in the NH₄Cl solution.