18: pH Flashcards
What the basicity of an acid
The number of H† ions that can be released from 1 molecule of the acid
Give an example of a monobasic acid
HCl
Give an example of a dibasic acid
H₂SO₄
What is the equation to work out pH when you have [H†]
pH = -log[H†]
How to work out [H†], given pH
[H†] = 10^-pH
What is [H†] equal to for a strong acid
The concentration of the acid
What is the equation for Kw
Kw = [H†][OH¯]
How to work out the pH of pure water
[H†]=[OH¯]
So Kw = [H†]²
[H†] = (1.00x10¯¹⁴)^½
-log[H†] = 7.00
How to work out the pH of strong alkalis
The [OH¯] = conc. of base
Rearrange Kw for H†
[H†] = Kw / [OH¯]
pH = -log[H†]
What is the general Ka equation
[H†][A¯] / [HA]
What does the value of Ka tell you about the strength of an acid
The larger the value of the Ka, the higher the H† conc., so the stronger the acid
What is the equation for working out pKa from Ka
pKa = -logKa
How to find Ka from pKa
Ka = 10^-pKa
What assumptions are made when calculating the pH of a weak acid
-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
What is the rearrangement of
Ka = [H†][A¯] / [HA]
For [H†] given some assumptions about weak acids
[H†] ≈ (Ka[HA]) ^½
What are the limitations of assuming that [H†] = [A¯] when calculating pH of a weak acid
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
What are the limitations of assuming that [HA] = total conc. of the acid
When calculating pH of a weak acid
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
What is a buffer solution
A system that minimises pH changes on addition of small amounts of acid or base
What does a buffer solution contain
A mixture of a weak acid and its conjugate base
What happens in a ethanoic acid buffer solution when a small amount of acid is added
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
What happens in a ethanoic acid buffer solution when a small amount of alkali is added
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
2 methods of preparing buffer solutions
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.
How to work out the pH of a buffer solution
Write the Ka expression for the the acid
Eg. Ka = [H†][CH₃COO¯]/[CH₃COOH]
Rearrange for [H†]
Plug in values
pH = -log[H†]
General rearrangement for [H†] for a buffer based on a weak acid
[H†] = Ka x [HA]/[A¯]
When is a buffer solution most effective
When it’s pKa is close to the pH
What range of pH is a buffer solution useful for
About 1 pH unit either side of pKa
What is in a basic buffer
A weak base and it’s conjugate acid
What is the buffer solution that is in blood
H₂CO₃ and HCO₃¯
How is H2CO3 formed in the body
When CO₂ dissolves in water
H₂O + CO₂ → H₂CO₃
