CK-12-Chemistry Intermediate

21.3. Acid and Base Strength http://www.ck12.org

Since one of the products of the ionization reaction is the hydroxide ion, we need to first find the value of [OH−] at
equilibrium. The pOH is 14 –12.31 = 1.69. [OH−] is then calculated to be 10−^1.^69 = 2.04× 10 −^2 M. The ICE table
is then set up as shown below (Table21.8).

TABLE21.8:ICE Table

Concentrations [C 2 H 5 NH 2 ] [C 2 H 5 NH 3 +] [OH−]

Initial 0.750 0 0

Change −2.04× 10 −^2 +2.04× 10 −^2 +2.04× 10 −^2

Equilibrium 0.730 2.04× 10 −^2 2.04× 10 −^2

Substituting into the Kbexpression yields the Kbfor ethylamine.

Kb=

[C 2 H 5 NH+ 3 ][OH−]

[C 2 H 5 NH 2 ]

=

( 2. 04 × 10 −^2 )( 2. 04 × 10 −^2 )

0. 730

= 5. 7 × 10 −^4

Calculating the pH of a Weak Acid or Weak Base

The Kaand Kbvalues have been determined for a great many acids and bases, as shown in the Acid and Base
Ionization Constants Tables (Table21.5 andTable21.6). These can be used to calculate the pH of any solution of
a weak acid or base whose ionization constant is known.

Sample Problem 21.5: Calculating the pH of a Weak Acid

Calculate the pH of a 2.00 M solution of nitrous acid (HNO 2 ). The Kafor nitrous acid can be found in the table
above (Table21.5).

Step 1: List the known values and plan the problem.

Known

• initial [HNO 2 ] = 2.00 M


• Ka= 4.5× 10 −^4

Unknown

• pH =?

First, an ICE table is set up with the variable x used to signify the change in concentration of the substance due to
ionization of the acid. Then, the Kaexpression is used to solve for x and calculate the pH.

Step 2: Solve.

TABLE21.9:ICE Table

Concentrations [HNO 2 ] [H+] [NO 2 −]

Initial 2.00 0 0

Change −x +x +x

Equilibrium 2.00−x x x

The Kaexpression and value is used to set up an equation to solve for x.