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Selasa, 02 Desember 2014

20.09    No comments

Hubungan Ksp dengan pH


Harga pH dapat digunakan untuk menghitung Ksp suatu basa yang sukar larut atau sebaliknya harga Ksp suatu  basa dapat digunakan untuk menentukan pH larutan.
      Contoh :
      1.  Larutan jenuh Zn(OH)2 mempunyai pH = 9. Hitunglah Ksp Zn(OH)2.
           Jawab :   PH = 9   maka POH = 14- 9 = 5
                          [OH-] = 10-5 M.
                         Zn(OH)2 ® Zn2+ + 2OH- ,
                          maka [Zn2+] = ½ x 10-5 M = 5x10-6 M.
                         Ksp Zn(OH)2 = [Zn2+][OH-]2    
                                               = (5 x 10-6)(10-5)2  = 5 x 10-16
      2.  Hitunglah kelarutan Mg(OH)2(Ksp = 3 x 10-11)dlm larutan jenuh memiliki pH =12.
           Jawab : pH = 12 Þ pOH = 14 –12 = 2 
                                                  [OH-] = 10-2 M.
           Ksp Mg(OH)2  = [Mg2+][OH-]2+ 
                             3 x 10-11   = [Mg2+][ 10-2]2  
                      [Mg2+]  = 3 x 10-7 M.

     3.   Diketahui Ksp Mg(OH)2 = 6 x 10-12.
           Tentukan  :
a.    kelarutan Mg(OH)2 dalam air 
b.    pH larutan  Mg(OH)2.

           Jawab : a. s = 1,14 x 10-4 Mol/liter.
b.   Mg(OH)2 ® Mg2+ + 2 OH-‑.
      [OH-] = 2 x 1,14 x 10-4‑M  = 2,28 x 10-4M
                             pOH = - log 2,28 x 10-4 = 4 – log 2,28          
                             pH = 10 + log 2,28 = 10,36.

4.    pH larutan jenuh  M(OH)2 = 9.
      Tentukan: a. kelarutan  M(OH)2 dalam air.       b.  Ksp M(OH)2 .
          Jawab : a. pH = 9 berarti pOH = 14 – 9 = 5 Þ [OH-] = 10‑5M.
                           M(OH)2 ® M2+  + 2 OH-    
                           [M2+] = ½  x 10-5 M. Maka kelarutan M(OH)2 = ½ x 10-5 M = 5 x 10-6 M.
                       b. Ksp M(OH)2 = [M2+][OH-]2
                                                 = ( 5 x 10-6)(10-5)2 = 5 x 10-16.
                                  
5.    Diketahui kelarutan Mg(OH)2 dalam 100 ml larutan sama dengan 0,58 mgram.. 
      Tentukan :
a.    pH larutan jenuh Mg(OH)2.   
b.    Ksp Mg(OH)2.

                  Jawab : 
                 a.  Mg(OH)2® Mg2+ + 2OH-  Þ [OH-] = 2 x 10-4 M.
                      POH = 4 – log 2   
                      maka pH = 14- (4-log 2) = 10 + log 2 = 10,3.
                 b. Ksp Mg(OH)2 = [Mg2+][OH-]2   = (10-4)(2 x 10-4)2.  
                     Maka  Ksp Mg(OH)2= 4x10-12.

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