Question

Natalie Predicting the qualitative acid-base propertie... Consider the following data on some weak acids and weak bases: acid base Ка Къ formula formula name name HCH CO2.8 x 10 5 CHN 9 1.7 x 10 acetic acid pyridine ethylamine C2H5NH2|6.4x 10 -4 HCIO hypochlorous acid 3.0 x 108 Use this data to rank the following solutions in order of increasing pH. In other words, select a 'l' next to the solution that will have the tF the solution that will have the next lowest pH, and so on. solution pH choose one 0.1 м КСI 0.1 M CSHSNНСІ choose one | 0.1 М КCНЗСО2 choose one choose one 0.1 М C2HsNнзBr

Answer 1

Since KCl is a salt of strong acid and strong base, this will have neutral pH. Hence the pH = 7.0

Since C₅H₅N is a base, its conjugate acid C₅H₅NH⁺Cl^- is an acid.

Ka of C₅H₅NH⁺Cl^- = Kw/Kb = 10^-14 / 1.7*10^-9 = 5.88*10^-6

Since C₂H₅NH₂ is a base, its conjugate acid C₂H₅NH₃⁺Br^- is an acid.

Ka of C₂H₅NH₃⁺Br^- = Kw/Kb = 10^-14 / 6.4*10^-4 = 1.56*10^-11

Since C₅H₅NH⁺Cl^- has higher Ka value, C₅H₅NH⁺Cl^- is more acidic than C₂H₅NH₃⁺Br^-.

i.e pH of C₅H₅NH⁺Cl^- is smaller than that of C₂H₅NH₃⁺Br^-, however since both are acid, their pH is less than 7.0.

K⁺CH₃CO₂^- is a conjugate base of weak acid HCH₃CO₂, its solution is basic. Hence pH > 7.

Since K⁺CH₃CO₂^- is the only basic solution, it has highest pH (pH>7), then KCl is the next lower one(pH = 7).

Since pH of C₅H₅NH⁺Cl^- is smaller than that of C₂H₅NH₃⁺Br^-, C₅H₅NH⁺Cl^- has the smallest pH.

Hence the order or increasing pH is:

0.1 M C₅H₅NHCl (smallest pH) < 0.1 M C₂H₅NH₃Br < 0.1 M KCl < 0.1 M KCH₃CO₂ (highest pH)