Weak acids and KaAQA A-Level Chemistry: Revision notes
Section 1
Weak acids and weak bases
A weak acid dissociates only slightly in water, so most of the acid stays as undissociated HA: HA ⇌ H⁺ + A⁻. A weak base such as ammonia accepts protons only slightly: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
A strong acid is fully dissociated. 'Weak' and 'strong' describe the extent of dissociation, whereas 'dilute' and 'concentrated' describe the amount of acid in the solution.
Do not confuse strong with concentrated. A dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid, but it is still a strong acid.
Section 2
The acid dissociation constant, Ka
For HA ⇌ H⁺ + A⁻ the acid dissociation constant is
Ka = [H⁺][A⁻] / [HA]
with units mol dm⁻³. Water is left out because its concentration is effectively constant. A larger Ka means a stronger weak acid. At 298 K, ethanoic acid has Ka = 1.74 × 10⁻⁵ mol dm⁻³.
pKa = –log₁₀Ka, so Ka = antilog(–pKa). A lower pKa means a stronger acid.
Convert between Ka and pKa with your calculator logarithm function, and check that a smaller Ka gives a larger pKa.
Section 3
pH of a weak acid
Two assumptions simplify the calculation:
- [H⁺] = [A⁻], because the acid is the only significant source of H⁺ (water contributes a negligible amount).
- [HA] at equilibrium ≈ its initial concentration, because the acid is only slightly dissociated.
Then Ka = [H⁺]² / [HA], so [H⁺] = √(Ka × [HA]).
Worked example. 0.100 mol dm⁻³ ethanoic acid: [H⁺] = √(1.74 × 10⁻⁵ × 0.100) = 1.32 × 10⁻³ mol dm⁻³, so pH = 2.88.
Using [H⁺] = [HA] as for a strong acid. For a weak acid [H⁺] is much smaller than the acid concentration.
Section 4
Finding Ka or concentration from a pH
Reverse the calculation. From a measured pH, find [H⁺] = antilog(–pH), then use the same assumptions.
Finding Ka. 0.150 mol dm⁻³ HA with pH 2.85: [H⁺] = 1.41 × 10⁻³ mol dm⁻³, so Ka = (1.41 × 10⁻³)² / 0.150 = 1.33 × 10⁻⁵ mol dm⁻³.
Finding concentration. [HA] = [H⁺]² / Ka.
Section 5
Comparing strong and weak acids
At the same concentration a weak acid has a lower [H⁺] and so a higher pH than a strong acid. For 0.100 mol dm⁻³ solutions, HCl has pH 1.00 and ethanoic acid has pH 2.88.
A larger Ka (smaller pKa) means a stronger weak acid. Ka depends on temperature, so quote the temperature when giving a value.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Weak acids and Ka
- Methanoic acid, HCOOH, is a weak acid found in ant stings. In aqueous solution it dissociates slightly: HCOOH ⇌ H⁺ + HCOO⁻. At 298 K its acid dissociation constant, Ka, is 1.78 × 10⁻⁴ mol dm⁻³. A solution of methanoic acid has a concentration of 0.0500 mol dm⁻³.Calculate the pH of the 0.0500 mol dm⁻³ solution of methanoic acid.2 marks
- A chemist measures the pH of a 0.150 mol dm⁻³ solution of a weak monoprotic acid, HA, at 298 K and finds it to be 2.85.State two assumptions made when calculating Ka for HA from the pH of its solution.2 marks
- Ethanoic acid, CH₃COOH, is the weak acid in vinegar. At 298 K its acid dissociation constant, Ka, is 1.74 × 10⁻⁵ mol dm⁻³.Calculate the pH of a 0.250 mol dm⁻³ solution of ethanoic acid. State the assumptions that you make.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).