The titration of a weak base ( K _b = 1.0×10^{- 4}) with HCl requires 40.0 mL to reach the equivalence point. In order to calculate the pH of the system after 20 mL of HCl is added, the best strategy is to recognize that the system is

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At the equivalence point, all of the weak base has been neutralized by the strong acid, resulting in a solution containing only the conjugate acid of the weak base. Therefore, at the equivalence point, the solution is acidic with a pH less than 7.

However, before the equivalence point, when only 20.0 mL of the HCl has been added, the system is still a weak base and the addition of the HCl will create a buffer system. The weak base will react with the HCl to form its conjugate acid, which will then act as a buffer to resist changes in pH. Therefore, the best choice is C, a buffer.

The Titration of a Weak Base ( K b =

The Titration of a Weak Base ( K _b =