## Acids and Bases

Definitions of Acid/Base

 Acid Base Bronsted-Lowry Donates proton Accepts proton Arrhenius Donates proton Donates OH- (hydroxyl) group Lewis Accepts electron pair Donates electron pair

pH and pOH

pH = -log10[H+]

pOH = -log10[OH-]

Kw

Water dissociates as follows:
H2O « H+ + OH-

Kw = [H+] [OH-] = 10-14 (at 25oC)
\pKw = pH + pOH = 14

(e.g. A solution with pH=9 has pOH = 14 - 9 = 5)

\In neutral water, [H+] = [OH-] = Ö(10-14) =10-7
pH = -log10[H+] = 7.0
pOH = -log10[OH-] = 7.0

Ka

An acid dissociates in water as follows:
HA « H+ + A-

Ka = [H+] [A-] / [HA]
pKa = -log10Ka

Kb

A base reacts with water as follows:
A- + H2O « HA + OH-

Kb = [HA] [OH-] / [A-]

(Note: the concentration of H2O is usually ignored)
pKb = -log10Kb

Note, Ka Kb = ([H+] [A-] / [HA]) x ([HA] [OH-] / [A-])
= [H+] [OH-]
= Kw
= 10-14

And since Ka Kb = 10-14
pKa + pKb = 14 = pKw

Henderson-Hasselbalch Equation
Ka = [H+] [A-] / [HA]
[H+] = Ka [HA]/[A-]
pH = pKa - log10[HA]/[A-]
pH = pKa + log10[A-]/[HA]
Buffers
Buffers are solutions that resist changes in pH when small amounts of acid or base are added to it.

A buffer usually consists of:

a weak acid and its conjugate base or
a weak base and its conjugate acid.
Such systems have the greatest buffering capability when the two components are in equal concentration.
(e.g. an acetic acid/acetate buffer has greatest buffering capability when [acetic acid] = [acetate], and therefore when
pH = pKa + log10[acetate]/[acetic acid]
= pKa + log10(1)
= pKa + 0
= pKa = 4.74)

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