17.2.1 An Introduction to Buffers

Question 1

An Introduction to Buffers

Answer 1

• The human body is a complex system of buffers. Without buffers the macroscopic characteristics of proteins would break down.
• Buffers are composed of a weak acid and its conjugate base.
• Calculations for buffer systems are acid-dissociation constant (K a ) problems.

Question 2

note

Answer 2

• The human body contains many complex systems involving proteins with various side chains. Among these are carboxylic acid groups and amino groups (fig. 1).
• In an equilibrium between acetic acid and ammonia, a proton can be transferred from the acetic acid to the amino group.
• At a blood pH of 7.5, the carboxylic acid groups are
  deprotonated. Also, the amino groups are protonated (fig. 2). These charged groups help hold proteins together and are responsible for much of the macroscopic structure of proteins. If the blood pH were to drop significantly below 7.5, the carboxylic acid groups would be protonated and the amino groups would be deprotonated. The entire structure of proteins in the blood would break down.
• Buffers are composed of a weak acid (HA) and a weak base (A – ). These acids and bases are conjugate acid-base pairs.
• If a small amount of acid is added to a buffered system, it will react with the weak base portion of the buffer. The H + concentration will not appreciably change and the pH will remain constant.
• If a small amount of base is added to a buffered system, it will react with the weak acid portion of the buffer. The H + concentration will not appreciably change and the pH will remain constant.
• Calculations for the pH of buffer systems are K a problems.
• Problem: A buffer is prepared using 1.00 M formic acid
  and 0.50 M formate ion. What is the pH of this solution at equilibrium?
• The weak acid (formic acid, HCOOH) will react with later to form formate ion (COOH – ). Formate ion concentration and H 3 O + concentration will increase by x and formic acid concentration will decrease by x.

Question 3

The human body is able to maintain a blood pH of about 7.5. Which statement is correct?

Answer 3

A radical change in blood pH could cause breakdown of blood proteins.

Question 4

Suppose that an acidic buffer solution consists of 1.00 M CH3COOH and 0.75 M Na(CH3CO2 ). Ka(for CH3COOH) is 1.75 × 10−5. What is the pH of this buffer solution?

Answer 4

4.63

Question 5

Let’s look at a buffer solution that contains a weak base, A−, and its conjugate acid, HA. Up to a certain point, there are two reactions that can occur:

Reaction 1: H+(aq) + A-(aq) <==> HA(aq)

Reaction 2: OH-(aq) + HA(aq) <==> A-(aq) + H2O(l)

Which statement about this buffer solution is not true?

Answer 5

In Reaction 1, [H + ] increases as it is added to the solution.

Question 6

Which of the following statements about a buffer solution is not true?

Answer 6

A buffer solution can maintain its pH no matter how much strong acid (or base) is added.

Question 7

Suppose that you have an acidic buffer (pH < 7) that is formed by adding 1.12 moles of formic acid (HCOOH, Ka = 1.77 × 10−4 ) and 0.89 moles of sodium formate (NaHCOO) to water to make 1 L of solution. The weak acid in this reaction is formic acid, HCOOH. Its conjugate base, derived by removing a H + ion, is HCOO−. What is the pH of the acidic buffer solution?

Answer 7

3.65

Question 8

Which statement best defines a buffer solution?

Answer 8

A buffer solution is a solution in which the pH does not change very much when a small amount of strong acid or strong base is added.

Question 9

Which of the following is not a conjugate pair?

Answer 9

HClO2 and ClO−

Question 10

A conjugate pair consists of two substances that are exclusively related by the gain or loss of a single hydrogen ion (H + ). Which of the following substance pairs is not a conjugate pair?

Answer 10

H3O and OH −

Question 11

Which of the following is not a buffer solution?

Answer 11

a strong acid (to neutralize a strong base) and its conjugate base (to neutralize a strong acid)