Acids, Bases and Buffers

Question 1

What is the Bronsted-Lowry definition of an acid?

Answer 1

Proton donor

Question 2

What is the Bronsted-Lowry definition of a base?

Answer 2

Proton acceptor

Question 3

What does acid-base equilibria involve?

Answer 3

The transfer of protons

Question 4

How do you find the pH of a strong acid?

Answer 4

pH = –log₁₀[H⁺]

Question 5

How do you find the H⁺ concentration from pH?

Answer 5

10⁻ᵖᴴ

Question 6

How dissociated is water?

Answer 6

Slightly dissociated

Question 7

What is the expression for the ionic product of water?

Answer 7

Kw = [OH⁻][H⁺]

Question 8

How do you calculate the pH of a strong base?

Answer 8

1) Kw = [OH⁻][H⁺]
2) Rearrange for [H⁺]
3) Substitute into pH = –log₁₀[H⁺]

Question 9

Define strong acids/weak acid

Answer 9

Fully dissociated in solution

Question 10

Define weak acid/weak base

Answer 10

Partially dissociated

Question 11

What is the dissociation constant for a weak acid?

Answer 11

Ka

Question 12

How is Ka related to Kc?

Answer 12

Kc [H₂O] = Ka

Question 13

What is the Ka expression for the equation:

CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

Answer 13

Ka = [CH₃COO⁻][H⁺] / [CH₃COOH]

Question 14

How do you calculate the pH of a weak acid?

Answer 14

Ka = [H⁺]² / [CH₃COOH]

• solve for [H⁺]
• use equation pH = –log₁₀[H⁺]

Question 15

How does temperature affect Ka?

Answer 15

When you increase temperature, you increase Ka

Question 16

What is the ‘end point’?

Answer 16

When the indicator changes colour

Question 17

What is the ‘equivalence point’?

Answer 17

When [H⁺] = [OH⁻]

Question 18

What indicator should be used for a strong acid/ weak base titration?

Answer 18

methyl orange

Question 19

What indicator should be used for a strong base/weak acid titration?

Answer 19

phenolphthalein

Question 20

How would you measure the pH change for a weak acid/weak base titration and why?

Answer 20

A pH probe because there is no sharp increase in pH.

Question 21

How to calculate the pH during a titration?

Answer 21

1) Calculate moles
2) Find whichever is in excess (x)
3) Find the concentration from the moles of x left
4) Find pH

Question 22

What is monoprotic?

Answer 22

Donates only one proton

Question 23

What is a buffer?

Answer 23

A buffer is able to resist changes to pH when moderate amounts of acid or base are added

Question 24

What makes up an acidic buffer?

Answer 24

Weak acid + Salt from the weak acid and strong base

Question 25

What equations are involved in acidic buffers?

Answer 25

1) HA ⇌ H⁺ + A⁻

2) NaA → Na⁺ + A⁻

Question 26

In acidic buffers, what happens when you add more acid (H⁺)?

Answer 26

The system opposes the addition of H⁺ by shifting the position of equilibrium to the left. It is able to do this because of the large reservoir of A⁻ (conjugate of base). Hence this ensures that the conc. of H⁺ remains almost constant.
H⁺ + OH⁻ → H₂O

Question 27

In acidic buffers, what happens when you add alkali (OH⁻)?

Answer 27

The system opposes the addition of OH⁻ by shifting the p.o.e to the right, it is able to do this because of the large reservoir of HA. Hence this ensures that the conc. of H⁺ remains almost constant.

Question 28

What makes a basic buffer?

Answer 28

Weak base + salt from weak base and strong acid

Question 29

What equations are involved in basic buffers?

e.g. NH₃ + NH₄Cl

Answer 29

1) NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

2) NH₄Cl → NH₄⁺ + Cl⁻

Question 30

In basic buffers, what happens when you add more acid (H⁺)?

Answer 30

The system opposes this by shifting the p.o.e to the right, it is able to do this because of the large reservoir of NH₃. Hence this ensures that the conc. of H⁺ remains almost constant.

Question 31

In basic buffers, what happens when you add alkali (OH⁻)?

Answer 31

The system opposes this by shifting the p.o.e to the left. It is able to do this due to large reservoir of NH₄⁺ (conjugate acid). Hence this ensures that the conc. of H⁺ remains almost constant.

Question 32

How do you calculate the pH of a buffer solution?

Answer 32

1) Calculate moles of HA (acid) and A⁻(salt) and find which is in excess
2) Find the conc. of both by using moles of excess left and original moles of the other.
3) Find the pH

Question 33

At half equvalence [HA]=?

Answer 33

[HA] = [A⁻]
pH = pKa