Exam 1 Results
 Average grade: 78
 Standard Deviation: 18
 High Grade: 105
Chapter 15.4
Qualitative Aspects of Weak Acids and Weak Bases
Objectives
◦Define weak acids and bases.
◦Relate Ka to the competition of different
bases for protons.
September 20th, 2016
Weak Acids and Weak Bases
 Weak acids and weak bases do not
ionize completely in water.
 A weak acid or base exists in equilibrium
with its conjugate partner.
HCN(aq) + H2O(l) ⇌ H3O+(aq) + CN-(aq)
Weak Acids and Weak Bases
 Weak acids and weak bases do not
ionize completely in water.
 A weak acid or base exists in equilibrium
with its conjugate partner.
HCN(aq) + H2O(l) ⇌ H3O+(aq) + CN-(aq)
acid
conjugate base
Weak Acids and Weak Bases
 Weak acids and weak bases do not
ionize completely in water.
 A weak acid or base exists in equilibrium
with its conjugate partner.
CH3NH2(aq) + H2O(l) ⇌ CH3NH3+(aq) + OH-(aq)
base
conjugate acid
Competition for Protons
 Both H2O and A- are bases and compete
for protons (H+).
100%
HCl(aq) + H2O(l)
H3O+(aq) + Cl-(aq)
 If HA is a strong acid, A- is an extremely
weak base (or proton accepter) and the
reaction will proceed in only one direction.
Competition for Protons
HF(aq) + H2O(l) ⇌ H3O+(aq) + F-(aq)
 If HA is a weak acid, A- is a weak base
and will accept protons easily. The
reaction will proceed in both directions as
an equilibrium.
Ka and Kb for Weak Acids and Bases
HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
H3O+ [A−]
Ka =
[HA]
B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)
BH+ [OH−]
Kb =
[B]
In Class Example
CH3COOH(aq) + H2O(l) ⇌ H3O+(aq) + CH3COO-(aq)
Acetic Acid
Acetate anion
Calculate the Ka and the pKa of acetic acid if
at equilibrium you measured the following
concentrations
[CH3CH2COOH] = 0.0987
[CH3CH2COO-] = 0.00133
[H3O+] = 0.00133
Chapter 15.5
Weak Acids
Objectives
◦Calculate acid ionization constants from
experimental data.
◦Write the chemical equation for the
ionization of a weak acid.
◦Calculate the concentrations of the
species present in a weak acid solution.
◦Calculate percent ionization from Ka and
September 20 , 2016
concentration.
th
In Class Example
 The pH of a 0.100 M solution of the weak
acid HOCl is 4.26. Calculate Ka and pKa
for the acid.
Student Example
 The pH of a 0.50 M solution of HOCl is
3.91. Calculate Ka and pKa for HOCl.
In Class Example
 Calculate the pH of a 0.50 M acetic acid
(CH3COOH) solution. Ka = 1.8 x 10-5
Student Example
 Calculate the pH of a 0.025 M solution of
HCN, Ka = 7.2  10-10.
Analytical Concentration of an Acid
 Analytical concentration: the total
concentration of all forms of an acid; both
the protonated and unprotonated forms.
HF(aq) + H2O(l) ⇌ H3O+(aq) + F-(aq)
Analytical concentration, CHF = [F-] + [HF]
Fraction Ionized
HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
[A−]
fraction ionized =
HA +[A−]
 Since [H3O+] = [A-] and [HA] + [A-] = CHA
fraction ionized
[H3O+]
=
CHA
Analytical Concentration of HA
Ionization Leads to Conductivity
In Class Example
 Calculate the fraction ionized for
0.500 M HOCl, Ka = 3.0 x 10-8.
Student Example
 Calculate fraction of HOCl ionized in
0.0500 M HOCl, Ka = 3.0 x 10-8.