Question

End of Chapter, Problem 62 Which would be a more effective buffer at pH 5.0 (see Table 2.4)? 10 mM acetic acid buffer or 10 mM HEPES buffer The 10 mM HEPES buffer because its pK is higher than the desired pH. The 10 mM acetic acid buffer because its pris closer to the desired pH. @ The 10 mM acetic acid buffer because its pk is less than the desired pH. The 10 mM HEPES buffer because its pK is closer to the desired pH. 10 mM acetic acid buffer or 20 mM acetic acid buffer O The 10 mM acetic acid buffer becatse lower a concentration provides greater buffering capacity. The 20 mM acetic acid buffer because a higher concentration provides greater buffering capacity The 20 mM acetic acid buffer because its pk is closer to the desired pH. The 10 mM acetic acid buffer because its pK is closer to the desired pH 10 mM acetic acid or 10 mM sodium acetate The 10 mM acetic acid and acetate solutions have equal buffering capacity, The 10 mM acetate solution because acetate is a weak base.
Answer all three please. The two following choices for the bottom one are:1. solutions containing only 10mM acetic acid or acetate cannot serve as buffers 2. The 10mM acetic acid solution acetic acid is a weak acid

Answer 1

An ideal buffer of a particular buffer concentration contains equimolar quantities of a weak acid/base and its salt to produce an acidic/basic buffer of maximum buffer capacity, respectively. The maximum buffer capacity of a buffer determines the highest concentration of acid or base that can be added to that particular buffer to cause a change in pH of the solution of 1 unit (+1 when base is added and -1 when an acid is added).

When such buffers are prepared, the Henderson-Hasselbach equation gives the pH of the buffer as pK_a + log([salt]/[acid]), with the pH becoming equal to pKa when the buffer is equimolar. So, compared to a HEPES buffer of pKa 3, the acetate buffer with pKa 4.75 will perform better as its maximum buffer capacity needs to be deviated from by adding only a small quantity of the salt.

Next, diluting a buffer does not affect its pH but when it comes to buffering across a range of pH, a greater ion concentration results in a better buffering capacity as it can associate or dissociate large number of ions to maintain the pH. This makes a 20mM acetate buffer better than a 10mM acetate buffer. It should also be noted that both being acetate buffers have the same pK values.

Finally, a buffer can be functional only when it is made of an acid/base and its salt as that system is what invokes a common ion effect, causing the buffer system to associate or dissociate according to the acid or base added to ensure a net pH of no change before and after the acid or base addition. Therefore, sodium acetate or acetic acid solution alone cannot act as a buffer nor will a solution of different acetate salts make a buffer.

So, summarizing, the correct options of the three questions are OPTION 2, OPTION 2 and OPTION 3.