Chapter 16
Acid-Base Titration and pH
Aqueous Solutions and the
Concept of pH

Self-ionization of water – 2 water
molecules produce a hydronium ion and a
hydroxide ion by transferring a proton
 H20

+ H20 H30+ + OH-
Concentration of hydronium ion and the
hydroxide ions are represented as [H30+]
and [OH-]
For pure water the [H30+] and [OH-] are
both 1.0 x 10-7 M
 Ion product constant for water (Kw) is
obtained by multiplying the [H30+] and
[OH-]

for room temperature is 1.0 x 10-14 M2, but
varies with temperature
 Kw
Neutral, acidic, and basic solutions
Neutral solutions have equal [H30+] and
[OH-]
 Acidic solutions have a greater [H30+] than
[OH-]
 Basic solutions have a lower [H30+] than
[OH-]

Calculating [H30+] and [OH-]
Kw = [H30+] x [OH-]
 Remember use Kw = 1.0 x 10-14 M2
 Ex: A 1.0 x 10-4 M solution of HNO3 has
been prepared. What is its [H30+]? What
is its [OH-]?

The pH scale

pH – the negative logarithm of the
hydronium ion concentration
 pH

= - log [H30+]
pOH – the negative logarithm of the
hydroxide ion concentration
 pOH

= -log [OH-]
pH + pOH = 14.0
pH less than 7 is acidic
 pH greater than 7 is basic
 pH equal to 7 is neutral

Calculations involving pH
Sig figs for pH are different because of the
logarithm
 There must be as many sig figs to the right
of the decimal for the pH as what there
were in the [H30+]

[H30+] is 1 x 10-7 has one sig fig and his
pH is 7.0
 Ex.

What is the pH of a solution if the [H30+] is
3.4 x 10-5 M?
pH = - log [3.4 x 10-5 M]
 pH = 4.47

Finding [H30+] from pH

pH = - log [H30+] can be rearranged to
solve for [H30+] by using antilog

[H30+] = 10-pH

Determine the hydronium ion
concentration of an aqueous solution that
has a pH of 4.0.

pH = -log [H30+]
4.0 = -log [H30+]
10-4.0 = [H30+]

The pH of a solution is determined to be
7.52. What is the pOH, [H30+], and [OH-]?

The molarity of strong acids and bases
can be used directly to calculate pH, but
not weak acids and weak bases because
they don’t ionize/dissociate completely;
instead pH must be measured and then
[H30+] and [OH-] calculated
Sect. 16-2: Determining pH and
Titrations

Acid-base indicators – compounds whose
colors are sensitive to pH
 Weak
acids or weak bases
 Different color in the ionized (In-) vs. nonionized form (HIn)
 HIn  H+ + In-
In an acid, accepts H+ to form HIn
 In a base, OH- combines with H+, so more
HIn ionizes to offset loss of H+, thus more
In- is present


Transition interval – the pH range over
which an indicator changes color
 If
a low pH, then the indicator is a stronger
acid
 If a higher pH, then the indicator is a weak
acid

pH meter – determines the pH of a
solution by measuring the voltage between
the two electrodes that are placed in the
solution

Titration – the controlled addition and
measurement of the amount of a solution
of known concentration required to react
completely with a measured amount of a
solution of unknown concentration
 Used
to determine equivalent volumes of acid
and base
 An example of a chemically equivalent
amount would be 1liter of 0.1M HCl reacting
with 0.1 mol solid NaOH
Equivalence point – the point at which the
two solutions used in a titration are
present in chemically equivalent amounts
 End point – the point in a titration at which
an indicator changes color


When choosing indicators to be used in a
titration remember:
 Strong
acid/strong base will have an
equivalence point at 7
 Strong acid/weak base will have an
equivalence point below 7
 Weak acid/strong base will have an
equivalence point above 7
Molarity and titration
Standard solution – the solution that
contains the precisely known
concentrations of a solute (“known”
solution)
 Primary standard – a highly purified solid
compound used to check the
concentration of the known solution in a
titration

Steps for determining molarity of
unknown solution in titration
1.
2.
3.
4.
Use balanced neutralization reaction to
determine chemically equivalent
amounts of acid and base
Determine moles of known solution
Determine moles of solute of unknown
solution (use stoichiometry)
Determine molarity of unknown
Example: In a titration, 27.4 mL of
0.0154M Ba(OH)2 is added to a 20.0 mL
sample of HCl solution of unknown
concentration. What is the molarity of the
acid solution?
 Ba(OH)2 + 2HCl  BaCl2 + 2H2O
1 mol
2 mol
1 mol 2 mol
