
J.R. S. answered 11d
Ph.D. University Professor with 10+ years Tutoring Experience
This is a buffer solution made up of a weak base (C6H5NH2) and the conjugate acid (C6H5NH3+). To find the ratio of these species, we can use the Henderson Hasselbalch equation:
pOH = pKb + log [C6H5NH3+] / [C6H5NH2] ... but first we need the pKb for C6H5NH2 (I find 9.42)
(a) pH = 3.90 so pOH = 14 - 3.90 = 10.1
10.1 = 9.42 +log (conj.acid/base)
log (conj.acid/base) = 0.68
[conj.acid]/[base] = 4.79
[C6H5NH2]/[C6H5NH3+] = 0.209
(b) pH = 4.47 so pOH = 9.53
9.53 = 9.42 + log [conj.acid]/[base]
log[conj.acid]/[base] = 0.11
[conj.acid]/[base] = 1.29
[base]/[conj.acid] = 0.775
If you want to do it directly to find base/conj.acid , i.e. [C6H5NH2]/[C6H5NH3] instead of doing it like the above examples which involves taking the reciprocal at the end, you can use the Ka for C6H5NH3+ (which is 4.58) and then use
pH = pKa + log [C6H5NH2] / [C6H5NH3+]
(c) pH = 4.58 = 4.58 + log [C6H5NH2] / [C6H5NH3+]
log [C6H5NH2] / [C6H5NH3+] = 0
[C6H5NH2] / [C6H5NH3+] = 1.0
(d) pH = 4.93 = 4.58 + log [C6H5NH2] / [C6H5NH3+]
log [C6H5NH2] / [C6H5NH3+] = 0.35
[C6H5NH2] / [C6H5NH3+] = 2.24