Acids and bases 2

Question 1

What is the pH of strong acids (3)

Answer 1

1. Strong acids completely ionise in solution
2. pH of 0.01 M HCl → pH = -log(0.01)
3. pH of 0.001 M H₂SO₄ → pH = -log(0.02), because of 1:2 ratio

Question 2

What is the pH of strong alkalis (2)

Answer 2

1. Strong alkalis completely ionise in solution
2. pH of 0.01 M NaOHpOH = -log(0.01)pOH = 2pH = 14 - 2pH = 12

Question 3

What is the pH of 3.0 x 10⁻³ M solution of the strong alkali Ba(OH₂) at 25℃

Answer 3

3.0 x 10⁻³ x 2, because 2 OH₂ ionises

pOH = -log(6.0 x 10⁻³)

pOH = 2.2

pH = 14 - 2.2

pH = 11.8

Question 4

What is the acid dissociation constant (4)

Answer 4

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

Question 5

What is pKa (5)

Answer 5

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 6

A weak acid HA has a pKa of 6. An alkali is added to give a dilute solution of HA to give a solution with pH 7. What proportion of HA is ionised → HA + H₂O ⇌ A⁻ + H₃O⁺

Answer 6

1. Adding alkali shifts the equilibrium to the left to reverse the change
2. More than 50% of reactants are reacted

Question 7

What is the pH of weak acid solutions

Answer 7

1. The pH of weak acids is always higher than strong of the same concentration due to their partial ionisation

Question 8

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

Answer 8

1. [H⁺] = ⎷Ka C
2. pH = -log([H⁺])

Question 9

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

Answer 9

[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 10

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

Answer 10

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

Question 11

What is the base dissociation constant (4)

Answer 11

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 12

What are the applications of acidity and basicity in drug design (6)

Answer 12

1. Acidity and basicity are important considerations in formulating drug products.
2. Many drugs are weak organic acids or bases, hence they will be ionised in solution.
3. Degree of ionisation is important in drug absorption
4. unionised compound are more hydrophobic and more likely to be absorbed through cell membranes.
5. The degree of ionisation depends on the pKa or pKb of the compound and the pH of the surroundings.
6. % ionisation = 100[H⁺] / C , works for pure acid solutions

Question 13

What is the Henderson-Hasslebalch equation (2)

Answer 13

1. pH = pKa + log([A⁻] / [HA])
2. % ionisation = 100 / 1 + 10⁽ᵖᴷᵃ ⁻ ᵖᴴ⁾

Question 14

What is the % ionisation of weak bases (1)

Answer 14

% ionisation = 100 / 1 + 10⁽ᵖᴴ ⁻ ᵖᴷʷ ⁺ ᵖᴷᵇ⁾

Question 15

What is the % ionisation and absorption (5)

Answer 15

1. A membrane, or lipid bilayer, is like a two-dimensional liquid composed of molecules with hydrophobic parts.
2. Unionised species passively cross membranes more easily as they are hydrophobic (= lipophilic).
3. Active transport uses carrier molecules to carry ions
4. Distribution = [HA]oil / [HA]water + [A⁻]water
5. Partition = [HA]oil / [HA]water