Acids and bases 2

Question 1

What is the acid dissociation constant (4)

Answer 1

1. A measure of acid strength
2. Low Ka = weak acid
3. High Ka = strong acid
4. Ka = [H₃O⁺][A⁻] / [HA]

Question 2

What is pKa (5)

Answer 2

1. Another measure of acid strength
2. pKa is the pH at which an acid is half ionised (when % ionisation is 50%)
3. Low pKa = strong acid (e.g. HCl → pKa = -6.3)
4. High pKa = weak acid (e.g.HCOOH → pKa = 3.8)
5. pKa = -logKa

Question 3

Why is the pH of weak acids always higher than strong acids of the same concentration

Answer 3

due to their partial ionisation

Question 4

How do you calculate the pH of weak acids (2)

Answer 4

1. [H⁺] = ⎷Ka C
2. pH = -log([H⁺])

Question 5

Estimate the pH of a 0.01 M solution of formic acid with Ka = 1.6 x 10⁻⁴

Answer 5

[H⁺] = ⎷Ka C
[H⁺] = ⎷1.6 x 10⁻⁴ x 0.01
[H⁺] = 1.3 x 10⁻³
pH = -log([H⁺])
pH = -log(1.3 x 10⁻³)
pH = 2.9

Question 6

What is the % ionisation of pure weak acids (3)

Answer 6

1. % ionisation = 100 x 10⁻ᵖᴴ / Conc.
2. % ionisation will increase if HO⁻ is added: it removes H⁺ and pulls the ionisation equilibrium to the right.
3. % ionisation will decrease if H⁺ is added: it pushes the ionisation equilibrium to the left.

Question 7

What is the base dissociation constant (4)

Answer 7

1. Kb = [BH⁺][H₃O⁻] / [B]
2. Weak bases have low Kb and high pKb.
3. Because weak bases have strong conjugate acids, you may also see base strength defined in terms of the acid dissociation constant of the conjugate acid, KBH⁺.
4. KBH⁺ = [B][HO⁺] / [BH⁺] ← conjugate acid dissociation constant

Question 8

What are the applications of acidity and basicity in drug design (3)

Answer 8

1. Drug formulation
2. unionised = hydrophobic → absorbed through cell membrane
3. Ionisation depends on pKa or pKb and the surrounding pH

Question 9

What is distribution

Answer 9

Distribution = [HA]oil / [HA]water + [A⁻]water

Question 10

What is partition

Answer 10

Partition = [HA]oil / [HA]water