Chemistry Acid and Base
Aqueous Acid/Base Chemistry
Resources: Harris ‘Quantitative Chemical Analysis’
Review: Pure water has a pH = 7 Autodissociation: H2O (( H3O+ + OH-
K = [H3O+][OH-]/[H2O] -log[H3O+] = 7 [H3O+] = 10-7 M = [OH-] [H2O] = 55.56 M K = 1.8 x 10-16 ; pKa = 15.74
pKa is the acid dissociation constant; low pKa (strong acid, high pKa (weak acid
we can also write Kw = [H3O+][OH-] Kw = 10-14 In water, pH + pOH = 14
pH scale
Strong Acids:
Complete dissociation of the acid in water:
HA ( H+ + A- note: H+ in H2O is hydrated (H3O+)
K (Ka since it is an acid) is large (For complete dissociation, K = infinity; however there is some very tiny amount that is not dissociated. For practical intents and purposes, dissociation is complete.), and pKa is a negative value. Ex: K = 108, pKa = -8
Example: hydrochloric acid (HCl)
What is the pH of a 0.13 M HCl solution in water?
HCl ( H+ + Cl-
0.13 M ( 0.13 M + 0.13 M
[H+] = 0.13 M pH = -log[H+] = -log(0.13) = 0.89 (very acidic)
Strong base:
Complete dissociation of strong base results in aqueous hydroxide ion (OH-).
CatOH ( Cat+ + OH-
K (Kb since it is a base) is large, but since the equilibrium is for a base, we use the value pKb to describe the degree of dissociation. Ka x Kb = Kw
Ex: sodium hydroxide, NaOH
What is the pH of a 0.003 M solution of NaOH?
NaOH ( Na+ + OH-
[OH-] = 0.003 M pOH = 2.52 pH = 14 – pOH = 11.48 (basic)
Weak acid:
If the acid does not dissociate completely in H2O, then it is a weak acid. (note: H2O does not dissociate completely, so it is a weak acid, and a weak base).
HA (( H+ + A-
Ex: acetic acid, CH3COOH
CH3COOH (( CH3COO- + H+
It is possible to calculate pH of a weak acid solution, knowing its concentration and its pKa (acid dissociation constant). pKa acetic acid = 4.76; Ka = 1.75 x 10-5 = [CH3COO-][H+]/[CH3COOH]
Calculate the pH of 0.15 M acetic acid in H2O. CH3COOH ((
Resources: Harris ‘Quantitative Chemical Analysis’
Review: Pure water has a pH = 7 Autodissociation: H2O (( H3O+ + OH-
K = [H3O+][OH-]/[H2O] -log[H3O+] = 7 [H3O+] = 10-7 M = [OH-] [H2O] = 55.56 M K = 1.8 x 10-16 ; pKa = 15.74
pKa is the acid dissociation constant; low pKa (strong acid, high pKa (weak acid
we can also write Kw = [H3O+][OH-] Kw = 10-14 In water, pH + pOH = 14
pH scale
Strong Acids:
Complete dissociation of the acid in water:
HA ( H+ + A- note: H+ in H2O is hydrated (H3O+)
K (Ka since it is an acid) is large (For complete dissociation, K = infinity; however there is some very tiny amount that is not dissociated. For practical intents and purposes, dissociation is complete.), and pKa is a negative value. Ex: K = 108, pKa = -8
Example: hydrochloric acid (HCl)
What is the pH of a 0.13 M HCl solution in water?
HCl ( H+ + Cl-
0.13 M ( 0.13 M + 0.13 M
[H+] = 0.13 M pH = -log[H+] = -log(0.13) = 0.89 (very acidic)
Strong base:
Complete dissociation of strong base results in aqueous hydroxide ion (OH-).
CatOH ( Cat+ + OH-
K (Kb since it is a base) is large, but since the equilibrium is for a base, we use the value pKb to describe the degree of dissociation. Ka x Kb = Kw
Ex: sodium hydroxide, NaOH
What is the pH of a 0.003 M solution of NaOH?
NaOH ( Na+ + OH-
[OH-] = 0.003 M pOH = 2.52 pH = 14 – pOH = 11.48 (basic)
Weak acid:
If the acid does not dissociate completely in H2O, then it is a weak acid. (note: H2O does not dissociate completely, so it is a weak acid, and a weak base).
HA (( H+ + A-
Ex: acetic acid, CH3COOH
CH3COOH (( CH3COO- + H+
It is possible to calculate pH of a weak acid solution, knowing its concentration and its pKa (acid dissociation constant). pKa acetic acid = 4.76; Ka = 1.75 x 10-5 = [CH3COO-][H+]/[CH3COOH]
Calculate the pH of 0.15 M acetic acid in H2O. CH3COOH ((