Download Chapter 19: Reactions of Acids and Bases

Section 1: The Self-Ionization of Water
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Water is amphoteric (?)
 As an acid water gives up a H+ ion to become an
OH- ion
 As a base water accepts a H+ ion to become an
H3O+ ion
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H2O + H2O
H3O+ + OH-
 In pure water at 25°C, both of these ions are found at
a concentration of 1 x 10-7
▪ Keq= [H3O+][OH-]/[H2O]2
▪ Keq x [H2O]2=[H3O+][OH-]
▪ Known as ion-product constant(Kw)= 1.0 x 10-14
 Kw= 1.0 x 10-14= [H3O+][OH-]
 This applies not only to pure water but to every water
solution at 25°C
 If we know the concentration of either the acid or
base in water solution, we can figure the other
concentration (see table 19-3 p. 627)
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Read through Sample Problem 1 and try the
practice problems
Extra Practice:
 1. What is the concentration of OH- ions in the
blood, if [H3O+]=4.5 x 10-8? Is blood acidic, basic,
or neutral?
 2. What is the concentration of OH- ions in
saturated lime if [H3O+] = 3.98 x 10-13M?

Soren Sorensen proposed a more compact way to express
[H3O+] known as the pH scale that says pH=-log[H3O+]
 pH is a measure of the acidity or basicity of a solution. It is
defined as the cologarithm of the activity of dissolved
hydronium ions (H3O+). Hydrogen ion activity coefficients
cannot be measured experimentally, so they are based on
theoretical calculations. The pH scale is not an absolute scale; it
is relative to a set of standard solutions whose pH is established
by international agreement
 Sorensen suggested the notation “PH” for convenience,
standing for “power of hydrogen”, using the cologarithm of the
concentration of hydrogen ions in solution, p[H]. Although this
definition has been superseded p[H] can be measured if an
electrode is calibrated with soltuion of known hydrogen ion
concentration.

Pure water is neutral with a pH of 7.0 at 25°C
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When calculating pH be careful wit Sig Figs
for any logarithm. The # of digits after the
decimal point should equal the # of sig figs in
the original number.
Example: log 8.7 x 10-4 calculator will say is
-3.0604807, but the original number only has
two sig figs so log should be rounded to 3.06
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1. What 2 ions are present in pure water?
What is the concentration of these ions?
2. What is the abbreviation and value of the
ion product constant of water?
3. What is the concentration of H3O+ ions in a
wheat flour and water solution if [OH-]=1.0 x
10-8M? Is wheat flour and water acidic, basic,
or neutral?
4. What is the equation for pH?
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Read through Sample Problem 2 and try the
practice problems
Extra Practice:
1. Lemon juice contains a weak acid. The
[H3O+] is 3.16 x 10-3. What is the pH?
2. Analysis of a sample of maple syrup reveals
that the concentration of OH- ions is 5.0 x 10-8
M. What is the pH of this syrup? Is it acidic,
basic, or neutral?

Two common ways
 Acid-base indicator-either weak acid or bases that
undergo a color change when they gain or lose a
H+
▪ In acidic solution litmus gains H+ ions and turns red
▪ In basic solution litmus loses H+ ions and turns blue
 Simple indicator equation
▪ HIn + H2O
H3O+ + In-
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A second more accurate way to measure pH
is to use a pH meter (see figure 19-7)