Strength of Acids
and Bases
Chapter 8
Section 8.4
The pH Scale
• We use this scale to measure
the strength of an acid or base.
• pH can use the concentration
of hydronium ions or hydrogen
ions
• pH can be measured with a pH
meter or an indicator with a
wide color range.
The pH scale
The pH scale is a way of
expressing the strength of
acids and bases. Instead of
using very small numbers,
we just use the NEGATIVE
power of 10 on the Molarity
of the H+/H3O+ (or OH-) ion.
Under 7 = acid
7 = neutral
Over 7 = base
pH Scale
pH of Common Substances
Acid – Base Concentrations
concentration (moles/L)
10-1
pH = 3
pH = 11
OH-
H3O +
pH = 7
10-7
H3O+ OH-
OH-
H3 O +
10-14
[H3O+] >
acidic
solution
[OH-]
[H3O+] = [OH-]
neutral
solution
[H3O+] <
[OH-]
basic
solution
Strong Acids and Bases
• Acids that ionize completely are
called strong acids.
• The strength of an acid is
determined by the amount of
ionization.
• Bases that dissociate almost
completely into its ions are called
strong bases.
List of Strong Acids
• HCl – hydrochloric acid
• HNO3 – nitric acid
• HClO4 – perchloric acid
• H2SO4 – sulfuric acid
List of Strong Bases
• NaOH – Sodium Hydroxide
• Ca(OH)2 - Calcium Hydroxide
• KOH – Potassium Hydroxide
• LiOH – Lithium Hydroxide
Weak Acids and Bases
• An acid that only slightly ionizes in a water
solution is called a weak acid.
• Weak acids have only a small percent of acid
molecules that donate their hydrogen, and most
remain the same.
• Weak acids have a higher pH than a strong acid
of the same concentration. (Remember
concentration is he amount of solute dissolved in
a given amount of solution)
• A base that dissociates only slightly in a water
solution is a weak base.
• In these reactions the equilibrium favors the
reactants over the products, so few ions form in
the solution.
List of Weak Acids and Bases
• C2H4O2 – Acetic Acid
• CH2O2 – Formic Acid
• NH3 - Ammonia
• NH4OH - Ammonium hydroxide
Buffers
• Buffers are solutions in which the pH
remains relatively constant, even when
small amounts of acid or base are added
• Buffered solutions contain either:
•
•
A weak acid and its salt
A weak base and its salt
Buffers
•
The operation of a buffer depends on a shift
in equilibrium to counteract any changes in
pH.
•
Buffers are used in products and processes
where the pH of the solution needs to be
controlled.
• Examples: Shampoo, Medicine, and
Fermentation
•
Our blood is a buffer, its pH is about 7.4
Electrolytes
• Substances whose molecules dissociate into
ions when they are placed in water.
• CATIONS (+)
ANIONS (-)
• Electrolyte is a substance that when dissolved
in water conducts electricity
• Strong electrolyte = completely ionized or
dissociated; strongly conductive solution
• Weak electrolyte = partially ionized or
dissociated; somewhat conductive solution
Electrolytes
• Many salts (ionic compounds) are
strong electrolytes
• Strong Acids are strong electrolytes
• Strong Bases are strong electrolytes
• In contrast, weak acids, weak bases are
weak electrolytes.