In the beginning of the titration the pH changes very little as base is added. A

Question

In the beginning of the titration the pH changes very little as base is added. As the base is added the salt NaC_2H_3O_2 is formed. C_2H_3O_2 is the conjugate base of HC_2H_3O_2. Since HC_2H_3O_2 is a weak acid then C_2H_3O_2 must be a relative good proto acceptor. Therefore, we have made a solution that contains a weak acid and its conjugate base. This solution is known as a buffer solution. Buffers solutions resist changes in pH when small amounts acid or bases are added to them or when they are diluted with H_2O. We can understand how buffers work by considering the equilibrium present in the solution. The chemical reaction which describes the equilibrium present in a buffer solution can be written in generic terms as HX(aq) + H_2O(I) doubleheadarrow H_3O^+(Aq) + X(aq) Here is HX is the generalized formula of a weak acid and X- is its conjugate base. The pH of a buffer solution can be calculated using the Henderson-Hasselbalch relationship pH- pK_a + log([base]/[acid] where pK_a = -logK_a and the terms [base] and [acid] refer to the concentrations of the pKa conjugate acid and base. The pK_a of the acid can be determined if the log term were to disappear, i.e. become zero. What would the quotient [base]/[acid] have to be for the log term to go to zero? Rights ... It's when the quotient equals 1. At this point pH = pK_a

Explanation / Answer

pH = pKa + log(base/acid)

If concentration of base is equal to that of the acid then the logarthmic ratio of (base/acid)becomes equal to 1.

At half equivalence point log(base/acid) becomes 1, then pH = pKa

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