Question

Sort the following titrations in order of increasing pH at the equivalence point.

LiOH tirated with HI, C5H5N titrated with HCl, HCN titrated with Ca(OH)2, HNO2 titrated with NaOH, CH3NH2 titrated with HBr

Answer 1

In the equivalence point:

a)

LiOH + HI = strong acid + strong base = neutrla pH; 7

b)

C5H5N,pyridine, pKb = 8.82 + HCl = strong acid + weak base; expect an acidic pH due to hydrolysis:

C5H5NH + H2O = C5H5N + H3O+, which is acidic, approx pH = 14-pKb = 14-8.82 = 5.18

c)

HCN is a weak acid + Ca(OH)2 strong base = expec thydrolysis

CN- + H2O = HCN + OH-

note that this is basic, therefore, expect a pH = 14-pKa =

d)

HNO2+ NaOH = NO2- in solution

NO2- + H2O = HNO2 + OH-; this is slightly basic

e)

CH3NH2 + HBrÇ; strong acid + weak base

CH3NH3+ + H2O = CH3NH2 + H3O+

The order:

acidic < neutral < basic

acidic solutions = C5H5N (pyridine) + CH3NH2 (ehtylamine) of this, pyridine is weaker base, therefore stronger acid than ehtylamine

basic solutions = HCN and HNO2, note that HCN is weaker acid than HNO2, therefore a better base CN,

most basic:

HCN titration > HNO2 titration

neutral solution

LiOH + HI neutral titration

acidic solution

CH3NH2 > HCN

then

order:

C5H5N < CH3NH2 < neutral LiOH + HI < HNO2 < HCN

acidic to most basic