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Which of the following statement is correct regarding an eqiumolar solution of a weak acid CH3COOH and a weak base NH4OH at 25C ?
  1. Higher the concentration of CH3COOH, lesser is the pH value.
  2. Higher the concentration of NH4OH, higher is the pH value.
  3. pH does not depend on the concentration of CH3COONH4.
  4. None of the above


Solution

The correct option is C pH does not depend on the concentration of CH3COONH4.
Theory:
For a salt of weak acid and weak base:
[H+]=KwKaKb
log[H+]=12logKw+12logKa12logKb
log[H+]=12logKw12logKa+12logKb
pH=12(pKw+pKapKb) valid only if h<0.1 or 10% 
pH=7+12(pKapKb) at 25oC
Since, expression for [H+] or pH does not contain any concentration term. Thus pH is independent of the concentration value.

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