Question

•Which of the following solutions will be the best buffer at a pH of 9.26? (Ka for HC2H3O2 is 1.8 x 10–5; Kb for NH3 is 1.8 x 10–5.)
a)0.20 M HC2H3O2 and 0.20 M NaC2H3O2
b)3.0 M HC2H3O2 and 3.0 M NH4Cl
c)0.20 M NH3 and 0.20 M NH4Cl
d)3.0 M NH3 and 3.0 M NH4Cl
e)3.0 M HC2H3O2 and 3.0 M NH3

actually, i used pH= pKa+ log(base/acid)
9.26=9.26+ log base/acid
I know [base]=[acid]
but how i can choose between 3.0M or 0.20M of NH3 ? (why answer is d not c?)

Answer 1

Both c and d give you pH 9.26 since base/acid = 1 and log 1 = 0. But d is the better answer because the more concentrated solutions make the buffer capacity larger; i.e., the more concentrated solutions can handle MORE of an added base and/or acid before changing more than +/- 1 pH.