Chapter 3: 3.4 Buffer Solutions

Question 1

What is a buffer solution?

Answer 1

A solution where the weak acid and conjugate base are present at the same time

Question 2

What requirements must be met for a solution to be a buffer solution?

Answer 2

1. Must contain a weak acid that will react with any OH- ions (from strong base)
2. Must contain weak base that will react with any H+ ions (from strong acid)
3. Acid and base in buffer solution must NOT react with each other

Question 3

What are the only species that can possibly qualify for a buffer solution?

Answer 3

A weak acid or base and its own conjugate partner

Question 4

When do buffers work best?

Answer 4

When there is about a 1:1 mixture of the weak species and its conjugate

Question 5

True or False:

Buffering function will work if the weak species and its conjugate are not in a 1:1 mixture

Answer 5

True, as long as the concentrations are within a factor 10, it works

Question 6

List:

Methods to produce an acid buffer

Answer 6

1. Mix the two required species together
2. Mix the weak acid and strong base together
3. Mix the strong acid and conjugate base together

Question 7

List:

Methods to produce a base buffer

Answer 7

1. Mix the two required species together
2. Mix the strong acid and weak base together
3. Mix the conjugate acid and strong base together

Question 8

State:

The first step in a buffer problem

Answer 8

1. Determine starting conditions

Direct reaction or mixing (produce/identify both buffer components)

Determine starting or after-mixing amounts/concentrations

Question 9

State:

The second step in a buffer problem

Answer 9

2. Analyze the equilibrium (common ion)

Write balanced equilibrium equation for K value

Question 10

Why do mole amounts also work in buffer solution problems?

Answer 10

1. The parent and conjugate species are always in the same volume
2. The value of x is always negligible when compared to the initial concentration of the parent and conjugate

Question 11

State the equation for:

Henderson-Hasselbalch Method

Answer 11

pH = pKa + log([A-]/[HA])

Question 12

How else can the Henderson-Hasselbalch Equation be expressed?

Answer 12

pH = pKa + log([n(A-)]/[n(HA)])

Question 13

What is the purpose of a buffer solution?

Answer 13

Resist change in pH
1. If strong acid is added, it reacts with weak base (conjugate base)
2. If strong base is added, it reacts with weak acid (conjugate acid)

Question 14

Define:

Acid-base titration

Answer 14

A method used to determine parameters such as the concentration of base (or acid) in a solution

Question 15

Define:

Equivalence point

Answer 15

Point in a titration, reached when stoichiometrically equivalent amounts of acid and base have been combined

Question 16

State the resulting pH of the resulting titrations

Answer 16

1. Strong acid-strong base: Solution is neutral (pH 7)
2. Weak acid-strong base: Solution is basic
3. Strong acid-weak base: Solution acidic

Question 17

What are acid-base indicators are used to detect?

Answer 17

Used to detect the equivalence point in a titration

Question 18

What is an indicator?

Answer 18

A weak organic acid that has a different colour than its conjugate base

Question 19

When [HIn] = [In-]…

Answer 19

• K(HIn) = [H3O+]
• pKHIn = pH
  pKHIn of the indicator is equal to the pH at which the colour change occurs

Question 20

Describe the ratio of the the indicator concentrations

Answer 20

1. At 1 pH unit below this pH or pKa value (acidic), indicator is about 90% in the HIn form
2. At 1 pH unit above this pH or pKa value (basic), indicator is about 90% in the In- form

Question 21

What is the point of the titration at which the indicator changes colour?

Answer 21

Endpoint

Question 22

What does a titration curve shows us?

Answer 22

Shows, in the form of a graph, the pH resulting from all the reactions occurring during a titration

Question 23

What is the characteristic of a strong acid-strong base titration curve?

Answer 23

A rapid increase in pH just before and after the equivalence point

Question 24

What is the pH of a weak acid-strong base titration determined by?

Answer 24

Determined by strong base

Question 25

At the equivalence point:

For a weak acid-strong base titration, the pH at equivalence point is…

Answer 25

Greater than 7, due to the conjugate base

Question 26

What is the characteristic of a weak acid-strong base titration curve?

Answer 26

• Smaller increase of slope near equivalence point
• Buffer region with midpoint

Question 27

At the equivalence point:

For a strong acid-weak base titration, the pH at equivalence point is…

Answer 27

Less than 7, due to conjugate acid

Question 28

What is the characteristic of a weak base-strong acid titration curve?

Answer 28

• Decreasing slope (as acidity increases)
• Smaller decrease of slope near equivalence point
• Buffer region with midpoint

Question 29

In a weak acid-strong base / weak base-strong acid titration, what does the midpoint on the titration curve indicate?

Answer 29

• pH = pKa
• [HA] : [A-] = 1 : 1