Chapter 18 - pH and Indicators

Question 1

What is the self-ionisation of water?

Answer 1

When water dissociates into ions H⁺ and OH⁻.

Pure water conducts electricity when it dissociates into its ions

Question 2

Where does the equilibrium lie in the dissociation of water into its ions?

Answer 2

The concentration of H⁺ and OH⁻ ions in pure water is very small so the equilibrium lies mainly on the left

Question 3

What is the expression of Kc for the dissociation of water into its ions?

Answer 3

[H⁺][OH⁻]
Kc = ————-
[H₂O]

Question 4

Why is the value of [H₂O] taken to be a constant?

Answer 4

Very little ionisation occurs

Question 5

Kw = …

Answer 5

Kw = Kc.[H₂O] = ionic product of water = [H⁺][OH⁻]

Question 6

What is the value of Kw at 25°C?

Answer 6

Kw = 1 x 10⁻¹⁴

Question 7

What is the value of [H⁺]?

Answer 7

[H⁺] = 1 x 10⁻⁷

Question 8

What happens if the conditions of a solution change (i.e, acid or base added) ?

Answer 8

If the conditions change, and the value of H⁺ increases (for example), then the value of OH⁻ must decrease by the same amount (since Kw is a constant)

Question 9

For acidic solutions, [H⁺] >[OH⁻]

Answer 9

[H⁺] > [OH⁻]

Question 10

For basic solutions [H⁺] > [OH⁻]

Answer 10

[H⁺] < [OH⁻]

Question 11

Is Kw temperature dependent?

Answer 11

Yes → the self-ionisation of water will increase at higher temperatures (as the forward reaction is exothermic) and the pH of pure water decreases at higher temperatures

Question 12

How do you calculate pH?

Answer 12

pH = -log₁₀[H⁺]

Question 13

How do you calculate pOH?

Answer 13

pOH = -log₁₀[OH⁻]

Question 14

How do you calculate the pH of a strong base?

Answer 14

pH = 14 - pOH

Question 15

What are the limitations of the pH scale?

Answer 15

○ Only applies to dilute solutions → over 1M the accuracy decreases
○ Scale only runs from 0-14, even though values outside this range exist
○ Only applies to aqueous solutions

Question 16

What is the difference between concentrated and strong?

Answer 16

A concentrated solution is one that contains a large amount of solute, whereas strong refers to the extent to which an acid can donate its H⁺ ions

Question 17

What symbol is used to represent the strength of an acid?

Answer 17

Ka (acid dissociation constant) - it gives a measure of the extent to which an acid is dissociated

Question 18

Do strong acids have a high or low value for Ka? Why?

Answer 18

Strong acids have a large amount of dissociation and a high value of [H⁺]. They therefore have a high value of Ka

Question 19

Do weak acids have a high or low value for Ka? Why?

Answer 19

Weak acids have small amount of dissociation and a small value of [H⁺]. They therefore have a low value for Ka

Question 20

What symbol is used to represent the strength of a base?

Answer 20

Kb - base dissociation constant

Question 21

How do you calculate the pH of a weak acid?

Answer 21

First, find the value of [H⁺] ions:

[H⁺] = √(Ka x Macid)

then put this figure into the original pH formula