STRENGTH OF ACIDS
AND BASES
&
SALTS AND BUFFERS
Chapter 19.3 and 19.5
Notes #21
Part 1: Strengths of Acids and Bases
How is there is a difference?
• Citrus fruits contain citric acid that we can eat.
• Industrial companies use sulfuric acid that can cause
severe burns to the skin.
• Why are some acids weak and some acids strong?
Determining the Strength of an Acid
• Use the acid dissociation constant (Ka) is a ratio:
Dissociated
Non-Dissociated
Weak acids have small
Ka values.
The stronger an
acid is, the larger is
its Ka value.
=Less dissociation/
Slightly ionize
=More dissociation/
Completely Ionize
Larger Ka = Stronger Acid
Strength of Bases
• Similar to acids, bases have the base dissociation
constant (Kb).
• Strong bases: dissociate completely into the hydroxide
ion and metal ion.
• Weak Bases: dissociate slightly into hydroxide ion and
conjugate acid.
Table of Relative Strengths
a. Explain
In the graph for the strong
acid, why are the heights of
H3O+ and A- bars the same
as the height of the HA bar?
a. Inferring
In the graph of the weak
acid, why is the height of the
H3O+ the same as the
distance from the top of the
second HA bar to the dotted
line?
Calculate the Ka of a weak acid.
Part 2: Buffers and Salts
What is a salt?
• It is a compound that contains an anion from an acid and
a cation from a base.
• They form as a result of an acid-base neutralization
reaction.
• Not just table salt! Although, this is also considered a salt.
HCl + NaOH  NaCl + H2O
KOH + HNO3  KNO3 + H2O
Solutions made from these salts can be
neutral, acidic, or basic.
Universal indicator solution has been added to
each of these 0.10M aqueous salt solutions.
NH4Cl
pH 5.3
NaCl
pH 7
CH3COONa
pH 5.3
Universal Indicator: Like a liquid pH strip. Changes the solution to a
color to tell you the pH.
Buffers
• A Buffer is made by making a solution that contains a
mixture of:
• a weak acid and its salt or
• a weak base and its salt.
Characteristics of a Buffer Solution:
• The pH of a buffer remains relatively constant
when small amounts of acid or base are added.
• The buffer capacity is the amount of acid or base
that can be added to a buffer solution before a
significant change in pH occurs.
Buffers Contd…
• Buffers have neutralizing “powers”.
• In a sense, it eats up the acid or base you have to remain
at a constant pH.
• Eventually, the buffer will be used up, and the pH can then
change dramatically.
How to choose a Buffer
• The pH of the acid or base being added to the buffer
should be within the pH range of the buffer.
Buffer System
pH Range
Monohydrogen
phosphate/dihydrogen phosphate
6.1-7.4
Ethanoate/ ethanoic acid
3.7-5.6
Carbonate/ hydrogen carbonate
9.2-11.0
Phosphate/ monohydrogen
phosphate
11.0-12.0
Question: Which of these buffers would be effective at pH 5.0?
Importance of Buffers
The chemical processes
inside a living cell are very
sensitive to pH.
Human blood is normally
maintained at a pH very
close to 7.4.
You have learned about
chemical processes that
ensure that the pH of blood
is kept near 7.4.
Buffer: Carbonic
Acid/bicarbonate
The Buffer in Our Blood
• An abnormal pH level of the blood can be due to several
factors: overexerting your body through exercise,
improper diet, drug consumption, and other biological
factors such as kidney failure (since the kidneys are
responsible for removing excess H+ ions and other
components of the pH buffer).
• Below 6.8 (acidosis) or above 7.8 (alkalosis) will result in
death.