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Buffer Solutions

Buffer is a solution that does not allow variation in pH on the addition of acid or alkali. A buffer may be prepared by taking a weak acid or a base and adding its conjugate salt to it, so that the  conjugate salt ensures a common ion composition. For example, a mixture of acetic acid and sodium acetate in certain molar  ratios acts as a buffer, which behaves as follows:

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Lionization of a weak acid follows the mass law which is expressed in terms of Henderson – Hasselbalch equation. If HA is a weak acid, then the dissociation equation is:

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According to the law of mass action, Ka, the dissociation constant of an acid is defined as:
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Solving for [H+] we get
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Taking logarithms of both sides we get

                                   log10 [H+] = log10Ka + log10
 (as log ab = log a +log b) multiplying throughout by negative sign ,

                                    - log10 [H+] = log10Ka – log10
- log10 [H+] = pH
  - log10 Ka    = pKa                 


    -log 10

  =  -log 10
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    .   .    pH = pKa + log10  
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In general

pH = pKa + log10  

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Conjugate acid and base represent the corresponding molar concentration, moles/1. Ionization of weak acids or weak bases in aqueous solutions is a reversible process.

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Its equilibrium constant K
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The concentration of water in aqueous solution is constant, hence its concentration (55.5moles/I) may be combined with K to give the ionization constant Ka:

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  H3O may be represented as H, hence the expression for K will be

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The numerical value of Ka  for acetic acid is 1.795 x 10-5.  For dilute aqueous solution of NH3, the equilibrium reaction is

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The ionization constant

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The numerical value of Kb for ammonia solution is 1.8 x 10-5 , which is almost identical to Ka.Hence weak acids and bases are ionized to the same extent in aqueous solutions.

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