Chemistry question-Need to clear confusion??

Question

Which of the following weak acid/conjugate base salt pairs would be the best choice for preparing a buffer with a pH of 5.25?

a)HCN/NaCn Ka=6.2e-10

b)KHSO3/K2SO3 Ka=6.0e-8

c)CH3CH2CO2H/NaCH3CH2CO Ka=1.3e-5

d)K2HC6H5O7/K3C6H5O7 Ka= 4.0e-5

I thought the answer was C but the correct answer is D. I just want to know how that's correct because using the formula -log(ka) i get a higher pH with C rather than D.

×

10

Answer 1

The one with a pKa closest to the desired pH of 5.25

a) pKa = 9.2

b) pKa = 7.2

c) pKa = 4.9

d) pKa = 4.4

I would have to agree with your answer. And for the record, even (d) isn't a great choice for pH 5.25 buffer.