Bases and Buffers
&
Titration Curves
Base dissociation constant (Kb), Buffer Solutions, Buffer Capacity, Acid-Base Titration Curves
SCH4U Grade 12 University Chemistry
Mr. Dvorsky
We learned in previous lessons that…

+
−
Ion product constant for water: 𝐾𝑤 = [𝐻3 𝑂𝑎𝑞
][𝑂𝐻𝑎𝑞
] = 1.0 × 10−14
o


+
−
In pure water at 25˚C, the [𝐻3 𝑂𝑎𝑞
] = [𝑂𝐻𝑎𝑞
] = 1.0 × 10−7
𝑚𝑜𝑙
pH = –log[H3O+] and pOH = –log[OH-]  pH + pOH = 14
+
𝐻𝐴(𝑎𝑞) + 𝐻2 𝑂(𝑎𝑞) ⇌ 𝐻3 𝑂(𝑎𝑞)
+ 𝐴−(𝑎𝑞)
𝐾𝑎 =
𝐿
[H3O+]=10-pH and [OH-]=10-pOH
+
[𝐻3 𝑂𝑎𝑞
][𝐴−𝑎𝑞 ]
[𝐻𝐴]
Base Dissociation Constant (Kb)
For weak bases:
+
−
𝐵(𝑎𝑞) + 𝐻2 𝑂(𝑎𝑞) ⇌ 𝐻𝐵(𝑎𝑞)
+ 𝑂𝐻(𝑎𝑞)
𝐾𝑒𝑞 =
+ ][𝑂𝐻 − ]
[𝐻𝐵𝑎𝑞
𝑎𝑞
[𝐵][𝐻2 𝑂]
But concentration of water is almost constant, so
𝐾𝑏 = 𝐾𝑒𝑞 [𝐻2 𝑂] =
+
−
[𝐻𝐵𝑎𝑞
][𝑂𝐻𝑎𝑞
]
[𝐵]
Steps for solving problems with weak bases are similar to those involving weak acids.
Acids and their Conjugate Bases
There is an important relationship between:
the dissociation constant for an acid, Ka,
and
+
−
𝐶𝐻3 𝐶𝑂𝑂𝐻(𝑎𝑞) + 𝐻2 𝑂(𝑎𝑞) ⇌ 𝐻3 𝑂(𝑎𝑞)
+ 𝐶𝐻3 𝐶𝑂𝑂(𝑎𝑞)
𝐾𝑎 =
the dissociation constant for its conjugate base, Kb.
−
−
𝐶𝐻3 𝐶𝑂𝑂(𝑎𝑞)
+ 𝐻2 𝑂(𝑎𝑞) ⇌ 𝑂𝐻(𝑎𝑞)
+ 𝐶𝐻3 𝐶𝑂𝑂𝐻(𝑎𝑞)
+
−
[𝐻3 𝑂𝑎𝑞
][𝐶𝐻3 𝐶𝑂𝑂𝑎𝑞
]
𝐾𝑏 =
[𝐶𝐻3 𝐶𝑂𝑂𝐻𝑎𝑞 ]
−
[𝑂𝐻𝑎𝑞
][𝐶𝐻3 𝐶𝑂𝑂𝐻𝑎𝑞 ]
− ]
[𝐶𝐻3 𝐶𝑂𝑂𝑎𝑞
So…
𝐾𝑎 × 𝐾𝑏 = (
+
−
[𝐻3 𝑂𝑎𝑞
][𝐶𝐻3 𝐶𝑂𝑂𝑎𝑞
]
[𝐶𝐻3 𝐶𝑂𝑂𝐻𝑎𝑞 ]
)(
−
[𝑂𝐻𝑎𝑞
][𝐶𝐻3 𝐶𝑂𝑂𝐻𝑎𝑞 ]
− ]
[𝐶𝐻3 𝐶𝑂𝑂𝑎𝑞
+
−
) = [𝐻3 𝑂𝑎𝑞
][𝑂𝐻𝑎𝑞
] = 𝐾𝑤
Thus, for an acid and its conjugate base, and for a base and its conjugate acid, K aKb=Kw
What does this mean?
The stronger the acid, the weaker its conjugate base must be.
So... The strength of an acid and its conjugate base are inversely related.
Buffer Solutions
Buffer Solution:
 A solution that contains a weak acid/conjugate base mixture or weak base/conjugate acid
mixture.
 Resists changes in pH when a moderate amount of acid or base is added to it
 Made two ways: Weak acid and one of its salts OR Weak base and one of its salts
 Characteristics of a buffer solution:
o pH
o buffer capacity
 amount of acid or base that can be added before considerable change to pH
 depends on concentration of acid/conjugate base or base/conjugate acid in buffer solution
 if more concentrated, resists changes to pH more than a buffer that is more dilute
 Blood has a buffer solution with hydrogen carbonate ions and carbonate ions, which is important to life.
How does a buffer solution resist changes in pH with the addition of an acid or a base?
Le Châtelier’s Principle
Acid-Base Titration Curves
Acid-Base Titration
 Common analytical procedure used to determine the concentration of a reactant, using pH, acids, bases and indicators
 Equivalence point
o point in a titration when the acid and base that are present completely react with each other
o middle of steep rise that occurs in titration curve
o If you know volume of both and concentration of one, you can find the concentration of the other.
 Endpoint
o Point in titration where indicator changes colour
o Pick an indicator that changes colour close to the equivalence point
Strong Acid-Strong Base
Titration Curve
pH = 7 at equivalence point
 Use indicators like
phenolphthalein, methyl red,
bromocresol green
Homework: Problem Sets Online
Weak Acid-Strong Base
Titration Curve
Weak Base-Strong Acid
Titration Curve
pH >7 at equivalence point
 Use indicators like
phenolphthalein
 Methyl red is not a useful
indicator
pH <7 at equivalence point
 Use indicators like methyl
red, bromocresol green
 Phenolphthalein is not a
useful indicato