Buffer weak base plus its salt with a strong acid

1 Buffer weak acid plus its salt with a strong base 

This buffer is prepared by mixing appropriate amounts of solutions of weak acid and its salt. Depending on the chemical nature of the weak acid, the resulting solution can have a pH which is acidic or basic. There are three equilibria which are simultaneously set up and must be simultaneously satisfied. 

If it can be assumed that the amount of HA(aq) removed by equilibrium(A) is approximately equal to that formed by equilibrium (B), then the change in [HA] due to the simultaneous setting up of both equilibria will be approximately zero, and so the approximation can be made that  

What is required is an expression for [H30 ]actuai- This can be found from either  

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