[ 1 ] 3.1.12 — Acids & Bases

Question 1

What is the Bronsted-Lowry definition of an acid?

Answer 1

A proton donor.

Question 2

What is the Bronsted-Lowry definition of a base?

Answer 2

A proton acceptor.

Question 3

What is a conjugate acid-base pair?

Answer 3

A pair of species that differ by one proton.

Question 4

What is the conjugate base of HCl?

Answer 4

Cl⁻

Question 5

What is the conjugate acid of NH₃?

Answer 5

NH₄⁺

Question 6

What does it mean for a species to be amphoteric?

Answer 6

It can act as both an acid and a base.

Question 7

What is the auto-ionisation of water?

Answer 7

2H₂O ⇌ H₃O⁺ + OH⁻

Question 8

What is the ionic product of water (Kw) at 25°C?

Answer 8

1.0 × 10⁻¹⁴ mol²dm⁻⁶

Question 9

How are [H⁺] and [OH⁻] related in water?

Answer 9

[H⁺][OH⁻] = Kw

Question 10

What is the pH of a neutral solution at 25°C?

Answer 10

7

Question 11

How is pH calculated?

Answer 11

pH = -log₁₀[H₃O⁺]

Question 12

How is [H⁺] calculated from pH?

Answer 12

[H⁺] = 10⁻ᵖᴴ

Question 13

What is a strong acid?

Answer 13

One that fully dissociates in water.

Question 14

Give an example of a strong acid.

Answer 14

HCl or HNO₃

Question 15

What is a weak acid?

Answer 15

One that partially dissociates in water.

Question 16

Give an example of a weak acid.

Answer 16

CH₃COOH (ethanoic acid)

Question 17

What is Ka?

Answer 17

The acid dissociation constant.

Question 18

How is Ka calculated?

Answer 18

Ka = [H₃O⁺][A⁻]/[HA]

Question 19

What is pKa?

Answer 19

pKa = -log₁₀Ka

Question 20

How do you calculate [H₃O⁺] for a weak acid?

Answer 20

[H₃O⁺] = √(Ka × [HA])

Question 21

What happens to the pH of a weak acid when diluted tenfold?

Answer 21

It increases by about 0.5 units.

Question 22

What is a polybasic acid?

Answer 22

An acid that can donate more than one proton.

Question 23

What is a strong base?

Answer 23

A base that fully dissociates into OH⁻ in water.

Question 24

Give an example of a strong base.

Answer 24

NaOH or Ba(OH)₂

Question 25

What is an alkaline solution?

Answer 25

A solution with more OH⁻ than H₃O⁺.

Question 26

How is pH of a strong base calculated?

Answer 26

Use [OH⁻], then [H⁺] = Kw/[OH⁻], then pH = -log[H⁺]

Question 27

What is a buffer solution?

Answer 27

A solution that resists changes in pH.

Question 28

What is a common buffer system?

Answer 28

CH₃COOH/CH₃COO⁻ or NH₄⁺/NH₃

Question 29

How does a buffer resist pH change when acid is added?

Answer 29

Base component neutralizes added H⁺.

Question 30

How does a buffer resist pH change when alkali is added?

Answer 30

Acid component neutralizes added OH⁻.

Question 31

What is the formula to calculate [H₃O⁺] in a buffer?

Answer 31

[H₃O⁺] = Ka × [acid]/[base]

Question 32

What is the Henderson-Hasselbalch equation?

Answer 32

pH = pKa + log([base]/[acid])

Question 33

When is a buffer most effective?

Answer 33

When [acid] = [base], so pH = pKa

Question 34

What is the pH of blood maintained at?

Answer 34

Around 7.4

Question 35

What is the function of HCO₃⁻ in blood?

Answer 35

It acts as a buffer to resist pH changes.

Question 36

What is an acid-alkali titration?

Answer 36

A neutralisation reaction to find concentration using an indicator.

Question 37

What happens at the equivalence point in a titration?

Answer 37

Acid and base are present in stoichiometric amounts.

Question 38

What is the pH at equivalence point for strong acid-strong base?

Answer 38

7

Question 39

What is the pH at equivalence point for weak acid-strong base?

Answer 39

8.5 approx

Question 40

What is the pH at equivalence point for strong acid-weak base?

Answer 40

5.5 approx

Question 41

What happens in a weak acid-weak base titration?

Answer 41

There is no sharp pH change.

Question 42

What is half-neutralisation?

Answer 42

The point where half the acid is neutralised and pH = pKa

Question 43

What is an acid-base indicator?

Answer 43

A weak acid that changes colour depending on pH.

Question 44

What happens at the indicator’s end-point?

Answer 44

[HIn] = [In⁻], so pH = pKa of the indicator.

Question 45

Give an indicator suitable for strong acid-strong base titration.

Answer 45

Phenolphthalein, methyl red, or litmus.

Question 46

Which indicator is suitable for weak acid-strong base titration?

Answer 46

Phenolphthalein.

Question 47

Which indicator is suitable for strong acid-weak base titration?

Answer 47

Methyl red.

Question 48

Why can’t indicators be used in weak acid-weak base titrations?

Answer 48

No clear pH jump at the equivalence point.