18: pH

Question 1

What the basicity of an acid

Answer 1

The number of H† ions that can be released from 1 molecule of the acid

Question 2

Give an example of a monobasic acid

Answer 2

HCl

Question 3

Give an example of a dibasic acid

Answer 3

H₂SO₄

Question 4

What is the equation to work out pH when you have [H†]

Answer 4

pH = -log[H†]

Question 5

How to work out [H†], given pH

Answer 5

[H†] = 10^-pH

Question 6

What is [H†] equal to for a strong acid

Answer 6

The concentration of the acid

Question 7

What is the equation for Kw

Answer 7

Kw = [H†][OH¯]

Question 8

How to work out the pH of pure water

Answer 8

[H†]=[OH¯]

So Kw = [H†]²

[H†] = (1.00x10¯¹⁴)^½

-log[H†] = 7.00

Question 9

How to work out the pH of strong alkalis

Answer 9

The [OH¯] = conc. of base

Rearrange Kw for H†

[H†] = Kw / [OH¯]

pH = -log[H†]

Question 10

What is the general Ka equation

Answer 10

[H†][A¯] / [HA]

Question 11

What does the value of Ka tell you about the strength of an acid

Answer 11

The larger the value of the Ka, the higher the H† conc., so the stronger the acid

Question 12

What is the equation for working out pKa from Ka

Answer 12

pKa = -logKa

Question 13

How to find Ka from pKa

Answer 13

Ka = 10^-pKa

Question 14

What assumptions are made when calculating the pH of a weak acid

Answer 14

-Assume [H†] = [A¯]
So Ka ≈ [H†]² /[HA]

-Assume that the undissociated acid concentration [HA], is equal to the total concentration of the acid, including those molecules that have dissociated

Question 15

What is the rearrangement of
Ka = [H†][A¯] / [HA]

For [H†] given some assumptions about weak acids

Answer 15

[H†] ≈ (Ka[HA]) ^½

Question 16

What are the limitations of assuming that [H†] = [A¯] when calculating pH of a weak acid

Answer 16

It ignores the H† ions formed by ionisation of the water solvent. This is reasonable provided that the number of H† from ionisation of the acid is much higher than the number that come from the water

Question 17

What are the limitations of assuming that [HA] = total conc. of the acid
When calculating pH of a weak acid

Answer 17

Ignores the fact that as some of the HA has broken up into H† and A¯, the actual concentration of the undissociated HA is less than the overall acid concentration

Question 18

What is a buffer solution

Answer 18

A system that minimises pH changes on addition of small amounts of acid or base

Question 19

What does a buffer solution contain

Answer 19

A mixture of a weak acid and its conjugate base

Question 20

What happens in a ethanoic acid buffer solution when a small amount of acid is added

Answer 20

CH₃COOH ⇌ CH₃COO¯ + H†

The increase in [H†] is opposed by the equilibrium shifting to the left, removing [H†] and forming CH₃COOH.

So [H†] doesn’t actually increase by much, and the pH is only slightly changed

Question 21

What happens in a ethanoic acid buffer solution when a small amount of alkali is added

Answer 21

It will react with the H† in the solution
(H† +OH¯ → H₂O)

The equilibrium:

CH₃COOH ⇌ CH₃COO¯ + H†

Will shift to the right to oppose the decrease in [H†]

Overall therefore, the [H†] doesn’t decrease by much and so the pH only increases very slightly

Question 22

2 methods of preparing buffer solutions

Answer 22

1) . Make a solution with a mixture of the weak acid and a salt of the acid
2) . Take a solution of the weak acid and partially neutralise it by adding an alkali, but not enough alkali to neutralise all of it.

Question 23

How to work out the pH of a buffer solution

Answer 23

Write the Ka expression for the the acid

Eg. Ka = [H†][CH₃COO¯]/[CH₃COOH]

Rearrange for [H†]

Plug in values

pH = -log[H†]

Question 24

General rearrangement for [H†] for a buffer based on a weak acid

Answer 24

[H†] = Ka x [HA]/[A¯]

Question 25

When is a buffer solution most effective

Answer 25

When it’s pKa is close to the pH

Question 26

What range of pH is a buffer solution useful for

Answer 26

About 1 pH unit either side of pKa

Question 27

What is in a basic buffer

Answer 27

A weak base and it’s conjugate acid

Question 28

What is the buffer solution that is in blood

Answer 28

H₂CO₃ and HCO₃¯

Question 29

How is H2CO3 formed in the body

Answer 29

When CO₂ dissolves in water

H₂O + CO₂ → H₂CO₃