CK-12-Chemistry Intermediate

21.5. Salt Solutions http://www.ck12.org

Again, the ratio of acetate ion to acetic acid changes slightly, this time causing a very small increase in the pH.

It is possible to add so much acid or base to a buffer that its ability to resist a significant change in pH is overwhelmed.
Thebuffer capacityis the amount of acid or base that can be added to a buffered solution before a large change in
pH occurs. The buffer capacity is exceeded when the number of moles of H+or OH−that are added to the buffer
exceeds the number of moles of the buffer components.

Some common buffer systems are listed below (Table21.13). Buffers are especially critical in biological systems.
For example, the pH of blood must be maintained within a fairly narrow range of about 7.35 to 7.45. There are
several buffers at work in blood, but the most important one involves carbonic acid (H 2 CO 3 ) and the hydrogen
carbonate ion (HCO 3 −). The ratio between the two is carefully maintained through metabolic processes so that the
pH of blood is nearly constant.

TABLE21.13: Some Common Buffers

Buffer system Buffer components pH of buffer (equal molarities of

both components)

Acetic acid / acetate ion CH 3 COOH / CH 3 COO− 4.74

Carbonic acid / hydrogen carbonate

ion

H 2 CO 3 / HCO 3 − 6.38

Dihydrogen phosphate ion / hydro-

gen phosphate ion

H 2 PO 4 −/ HPO 42 − 7.21

Ammonia / ammonium ion NH 3 / NH 4 + 9.25

Lesson Summary

• Salt solutions can be acidic, basic, or neutral. Certain ions can undergo acid-base reactions with water, causing
  a change in the pH of the resulting solution.


• Salts containing anions that are conjugate bases of weak acids can accept hydrogen ions from water to form
  their conjugate acid and the hydroxide ion. These solutions are slightly basic, and the pH can be calculated if
  the Kbof the anion is known.


• Salts containing cations that are conjugate acids of weak bases can donate hydrogen ions to water to form their
  conjugate base and the hydronium ion. These solutions are slightly acidic, and the pH can be calculated if the
  Kaof the cation is known.


• Salts formed by reacting a strong acid with a strong base do not have sufficiently acidic or basic components,
  so a solution containing this type of salt will have a pH of 7.


• A buffer is a solution of a weak acid or base together with one of its salts. Buffers resist changes in pH when
  additional acid or base is added.

Lesson Review Questions

Reviewing Concepts

1. What type of salt produces an acidic solution? A basic solution?


2. Predict whether an aqueous solution of the following salts will be acidic, basic, or neutral.
   a. K 2 CO 3
   b. KCl
   c. NH 4 NO 3