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184 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 18
C 6 H 5 NH 2 (aq) ϩ H 2 O(l) 3 C 6 H 5 NH 3
ϩ (aq) ϩ OH Ϫ (aq)
The equilibrium constant expression for the ionization of aniline in
water is as follows.
K b ϭ
K b , called the base ionization constant, is the value of the equilibrium constant expression for the ionization of a weak base. As you
might expect, K b is smallest for the weakest bases. Aniline is a very
weak base with K b ϭ 4.3 ϫ 10 Ϫ10 at 298 K.
Practice Problems
5. Write ionization equations and base ionization constant
expressions for the following bases.
a. butylamine (C 4 H 9 NH 2 )
b. phosphate ion (PO 4
3Ϫ )
c. hydrogen carbonate ion (HCO 3
Ϫ )
18.3 What is pH?
Pure water self-ionizes slightly to form H 3 O ϩ and OH Ϫ ions, as
shown in this equation.
H 2 O(l) ϩ H 2 O(l) 3 H 3 O ϩ (aq) ϩ OH Ϫ (aq)
The equation for the equilibrium can be simplified by removing one
water molecule from each side.
H 2 O(l) 3 H ϩ (aq) ϩ OH Ϫ (aq)
A special equilibrium expression for the self-ionization of water is
defined as follows.
K w ϭ [H ϩ ][OH Ϫ ]
K w , called the ion product constant for water, is the value of the
equilibrium constant expression for the self-ionization of water. In
pure water at 298 K, the concentrations of H ϩ ions and OH Ϫ ions
both equal 1.0 ϫ 10 Ϫ7 M, so the value of K w can be calculated.
K w ϭ [H ϩ ][OH Ϫ ] ϭ (1.0 ϫ 10 Ϫ7 )(1.0 ϫ 10 Ϫ7 )
K w ϭ 1.0 ϫ 10 Ϫ14
[C 6 H 5 NH 3
ϩ ][OH Ϫ ]
ᎏᎏᎏ
[C 6 H 5 NH 2 ]