Intro to the problem:

To understand how buffers use reserves of conjugate acid andconjugate base to counteract the effects of acid or base additionon pH.

A buffer is a mixture of a conjugate acid-base pair. In otherwords, it is a solution that contains a weak acid and its conjugatebase, or a weak base and its conjugate acid. For example, an aceticacid buffer consists of acetic acid, CH3COOH, and its conjugatebase, the acetate ion CH3COO−. Because ions cannot simply be addedto a solution, the conjugate base is added in a salt form (e.g.,sodium acetate NaCH3COO).

Buffers work because the conjugate acid-base pair work togetherto neutralize the addition of H+ or OH− ions. Thus, for example, ifH+ ions are added to the acetate buffer described above, they willbe largely removed from solution by the reaction of H+ with theconjugate base:

H++CH3COO−→CH3COOH

Similarly, any added OH− ions will be neutralized by a reactionwith the conjugate acid:

OH−+CH3COOH→CH3COO−+H2O

This buffer system is described by the Henderson-Hasselbalchequation

pH=pKa+log[conjugate base][conjugateacid]    

Part A

A beaker with 1.80×10² mL of an acetic acid bufferwith a pH of 5.000 is sitting on a benchtop. The total molarity ofacid and conjugate base in this buffer is 0.100 M. Astudent adds 5.60 mL of a 0.330 M HCl solution to thebeaker. How much will the pH change? The pKa of aceticacid is 4.740.

Express your answer numerically to two decimal places. Use aminus ( − ) sign if the pH has decreased.