Section Two

Chapter Eight, Section Two

OBJECTIVES

Understand the concept of self-ionization of water.
Know how to determine pH, pOH, and pK from several different starting points.
Know what aqueous salt solutions will be acidic, basic, or neutral.

I. Self-Ionization of Water

Perhaps surprisingly, pure water does contain a small amount of H₃⁺ and OH^- ions.

H₂O + H₂O H₃O⁺ + OH^-

Writing the equilibrium constant and then multiplying both sides by [H₂O]² gives

K_eq[H₂O]² = [H₃O⁺][OH^-]

Since the concentration of water is virtually constant, we can replace the left hand side with a new equilibrium constant, K_w, called the ion product of water:

K_w = [H₃O⁺][OH^-]

At room temperature, K_w = 1.0 x 10^-14. The autoionization of water produces both the ions so their concentrations must be equal, or

[H₃O⁺] = 1 x 10^-7

[OH^-] = 1 x 10^-7

K_w = [H₃O⁺][OH^-] applies to any water solution. However, the individual concentration of [H₃O⁺] and OH^- will change, but their product will always be 1 x 10^-14. Very important. Make sure that you understand this concept by working problem 8.2 on page 207.

II. pH, pOH, and pK

The p's are introduced for ease of communication of information about acid/base systems. One way to think about it is that the "p" stands for "-log[]". So pH = -log[H₃⁺], pOH = -log[OH^-], and pK = -logK. Remember that K_w = 1.0 x 10^-14, so that -logK_w = 14. But since K_w = [H₃O⁺][OH^-], AND since the log of a product of things is the sum of the logs {log(ab) = log(a) + log(b)}, we have that

14 = pH + pOH

Then if we know the pH we can easily calculate the pOH, or vice versa.

Acid solutions have a pH less than 7 and a basic solution has a pH greater than 7. The lower the pH, the more acidic the solution and the higher the pH, the more basic the solution. It is also important to understand that a change of one in the pH value means a ten-fold change in the [H₃⁺], which is considerable. We can obtain qualitative measures of the pH by using either pH paper or an indicator.

Some common indicators and the pH range over which they change color are:

We can obtain quantitative measures of the pH by using a pH meter as indicated in Figure 8.5 on page 211.

III. pH of aquous solutions of salts

Salts will be soluble in water, but will any of the resultant ions react with water? If one of the ions reacts with water and produces [H₃O⁺] then the resultant solution is acidic. If one of the ions reacts with water and produces [OH^-] then the resultant solution is basic. So we just need to know what happens when the salt dissociates in water and whether the resultant solution is acidic or basic will then be determined. We have four possibilities

A salt of a strong acid and a strong base produces a neutral solution. Examples of such salts are NaCl, KNO₃, and Na₂SO₄.
A salt of a weak acid and a weak base. This is a bit more difficult to predict and we will not be concerned with such cases.
A salt of a strong base and a weak acid produces a basic solution. (strong base ->basic)
A salt of a strong acid and a weak base produces an acidic solution. (strong acid ->acidic)

You have to then refer to our previous discussions to determine what sort of acid and base components of the salt we have.

ASSIGNMENTS

Work problems 8.5, 8.6, and 8.7 to make sure you understand these concepts.
Work problem 8.36 at the end of the chapter.
Go practice on pH problemss
Here are some problem sets for pH and other concepts.

QUIZ ONE

After you have studied this material and practiced some problems, take quiz two. If you score at least 80 on the test then you are ready to continue to the next section.