Honors Chemistry
Name: __________________________________________
Date: __________________
Calculating pH, Ka and Percentage of Ionization for Weak Acids
Weak Acids:
Most acidic substances are weak acids. Unlike strong acids, weak acids are only partially
ionized in aqueous solution. We can use the equilibrium constant for the ionization reaction to
express the extent to which a weak acid ionizes. If we represent a general weak acid as HA, we
can write the equation for its ionization in the space below.
We can then use this equation to write an equilibrium-constant expression for this reaction:
*Ka is called the acid-dissociation constant.
The magnitude of Ka indicates the tendency of the acid to ionize in water. If the value of Ka is
greater than one, the equilibrium lies to the right, and we are dealing with a ____________ acid.
If the value of Ka is less than one, the equilibrium lies to the left, and we are dealing with a
______________ acid. Therefore, the higher the value of Ka, the ________________ the acid.
Because weak acids only partially ionize in aqueous solution, we must use our skills for solving
equilibrium problems (ICE tables) from Chapter 15 in order to calculate either the Ka value for a
weak acid or the pH of its solutions.
Calculating Ka and Percent Ionization from Measured pH:
1. A student prepared a 0.10 M solution of formic acid (HCHO2) and measured its pH using
a pH meter. The pH at 25°C was found to be 2.38. (a) Calculate Ka for formic acid at
this temperature. (b) What percentage of the acid is ionized in this 0.10 M solution?
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Using Ka to Calculate pH:
2. Calculate the pH of a 0.30 M solution of acetic acid (HC2H3O2), the weak acid
responsible for the characteristic odor and acidity of vinegar, at 25°C. What percentage
of the acid is ionized in this 0.30 M solution? The Ka for acetic acid is 1.8 x 10-5.
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Polyprotic Acids:
Many acids have more than one ionizable H atom. These acids are known as polyprotic acids.
For example, each of the H atoms in sulfurous acid can ionize in successive steps. Write these
steps below.
The acid-dissociation constants for these equilibria are labeled Ka1 and Ka2. The numbers on
the constants refer to the particular proton of the acid that is ionizing. Thus, Ka2 always refers to
the equilibrium involving removal of the second proton of a polyprotic acid.
By viewing the Ka values listed above for the successive ionizations of the H atoms in sulfurous
acid, it is obvious that it is always _____________ to remove the first proton from a polyprotic
acid than to remove the second. For an acid with three ionizable protons, it is easier to remove
the second proton than the third. Thus, Ka values become successively ______________ as
successive protons are removed.
More Weak Acid ICE Table Practice Problems!
1) Lactic acid (HC3H5O3) has one acidic hydrogen. A 0.10 M solution of lactic acid has a
pH of 2.44. Calculate Ka and the percentage of the acid that is ionized.
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2) Phenylacetic acid (HC8H7O2) is one of the substances that accumulates in blood of
people with phenylketonuria, an inherited disorder that can cause mental retardation or
even death. A 0.085 M solution of HC8H7O2 is found to have a pH of 2.68. Calculate Ka
and the percentage of the acid that is ionized.
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3) Calculate the pH of a 0.20 M solution of HCN. What percentage of the acid is ionized in
this solution? The Ka for hydrocyanic acid is 4.9 x 10-10.
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4) Calculate the pH of a 0.50 M solution of benzoic acid (HC7H5O2). What percentage of
the acid is ionized in this solution? The Ka for benzoic acid is 6.3 x 10-5.
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Calculating pH, Kb and Percentage of Ionization for Weak Bases
Weak Bases:
Many substances behave as weak bases in water. Weak bases react with water, removing
protons from H2O to form the conjugate acid of the base and OH- ions. If we represent a
general weak base as B, we can write the equation for its ionization in the space below.
The most commonly encountered weak base is ammonia. The equation for its ionization is:
We can then use this equation to write an equilibrium-constant expression for this reaction:
*Kb is called the base-dissociation constant.
Calculating Kb and Percent Ionization from Measured pH:
1. Ephedrine, a central nervous system stimulant, is used in nasal sprays as a
decongestant. This compound is a weak organic base:
C10H15ON (aq) + H2O (l)  C10H15ONH+ (aq) + OH- (aq)
A 0.035 M solution of ephedrine has a pH of 11.33. (a) Calculate Kb for ephedrine.
(b) What percentage of the base is ionized in this solution?
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Using Kb to Calculate pH
2. Calculate the pH of a 0.15 M solution of NH3. The Kb for ammonia is 1.8 x 10-5. What
percentage of the base is ionized in this solution?
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The Relationship Between Ka and Kb:
The product of the acid-dissociation constant for an acid and the base-dissociation constant for
its conjugate base is equal to the ion-product constant for water as seen below:
Therefore, as the strength of an acid _________________ (larger Ka), the strength of its
conjugate base must ____________________ (smaller Ka) and vice versa so that the product
Ka x Kb always equals 1.0 x 10-14 at 25°C.
Example: The acid dissociation constant for hydrofluoric acid (HF) is 6.8 x 10-4. What is the
base-dissociation constant, Kb, for the fluoride ion (F-)?
This equation can also be written in terms of pKa and pKb by taking the negative log of both
sides as seen below.
More Weak Base ICE Table Practice Problems!
1. Codeine (C18H21NO3) is a weak organic base. A 5.0 x 10-3 M solution of codeine has a
pH of 9.95. (a) Calculate the value of Kb for this substance. (b) What percentage of the
base is ionized in this solution?
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2. Calculate the pH of a 0.075 M solution of ethylamine (C2H5NH2), a weak base. The Kb for
ethylamine is 6.4 x 10-4. What percentage of the base is ionized in this solution?
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