CK-12-Chemistry Intermediate

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

TABLE21.6: Base Ionization Constants at 25°C

Name of Base Ionization Equation Kb

Methylamine CH 3 NH 2 +H 2 O⇀↽CH 3 NH+ 3 +OH−5.6× 10 −^4

Ammonia NH 3 +H 2 O⇀↽NH+ 4 +OH− 1.8× 10 −^5

Pyridine C 5 H 5 N+H 2 O⇀↽C 5 H 5 NH++OH− 1.7× 10 −^9

Acetate ion CH 3 COO−+H 2 O⇀↽CH 3 COOH+OH5.6−× 10 −^10

Fluoride ion F−+H 2 O⇀↽HF+OH− 1.4× 10 −^11

Urea H 2 NCONH 2 +H 2 O⇀↽H 2 CONH+ 3 +OH1.5−× 10 −^14

Notice that the conjugate base of a weak acid is also a weak base. For example, the acetate ion has a small tendency
to accept a hydrogen ion from water to form acetic acid and the hydroxide ion.

Calculations with K

The numerical value of Kaor Kbcan be determined by experiment. A solution of known concentration is prepared,
and its pH is measured with an instrument called a pH meter (Figure21.8).

FIGURE 21.8

A pH meter is a laboratory device that

provides quick, accurate measurements

of the pH of solutions.

Sample Problem 21.4 shows the steps involved to determine the Kaof formic acid (HCOOH).

Sample Problem 21.4: Calculation of an Acid Ionization Constant

A 0.500 M solution of formic acid is prepared, and its pH is measured to be 2.04. Determine the Kafor formic acid.

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