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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: shehri on April 21, 2009, 09:44:09 PM

Title: Finding 'ph' value from 'Ksp'!!
Post by: shehri on April 21, 2009, 09:44:09 PM

Dear friends,

              Plz. guide me how to calculate 'ph' of saturated solution of Zn(OH)2 whose Ksp is 2.1X10^-16?Thanks in advance.

Title: Re: Finding 'ph' value from 'Ksp'!!
Post by: UG on April 21, 2009, 10:15:54 PM

K_s= [Zn²⁺][OH^-]²

x mol Zn(OH)₂   ::equil:: x mol Zn²⁺ + 2x mol OH^-

So K_s(2.1X10^-16)= x.(2x)²

x=3√(K_s/4)

Once you've got that you can calculate the solubility of OH^- in mol L^-1 and therefore the pH.

Title: Re: Finding 'ph' value from 'Ksp'!!
Post by: Loyal on April 22, 2009, 12:35:16 AM

Quote from: UG on April 21, 2009, 10:15:54 PM

K_s= [Zn²⁺][OH^-]²

x mol Zn(OH)₂   ::equil:: x mol Zn²⁺ + 2x mol OH^-

So K_s(2.1X10^-16)= x.(2x)²

x=3√(K_s/4)

Once you've got that you can calculate the solubility of OH^- in mol L^-1 and therefore the pH.

Actually in that situation you need to do more.  The amount of OH being produced is rather small so you need to take the hydrolysis of water into account.