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2015-03-21T16:35:32+05:30
The pH for a buffer solution is 
CH3COOH    <----------------------->   CH3COO-   +   H+
acid                                                 base

Ka =  [H+][CH3COO-]/[CH3COOH]
pKa = -log(1.8*10^-5)
pKa = 4.74

CH3COONa  ---------------------------->  CH3COO-  +  Na+
salt                                                  base

pH = pKa  +  log{[base]/[acid]}

salt will completely dissociate 
 acetic acid is a weak acid the concentration of [CH3COO-] is very less as compared to concentration of [CH3COO-] produces by salt.
as the dissociation constant of acid is very less hence 
[CH3COOH] is remains unchanged at equilibrium.  
pH = pKa + log{[CH3COO-]/[CH3COOH]}
as the concentration of CH3COO-  and CH3COOH  are same hence.
pH = 4.74 + log1
pH = 4.74
 
if you not understand please ask.

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