OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 17

9 marks · Hard difficulty · Extended Response

Explain how the carbonic acid-hydrogencarbonate buffer maintains blood pH and calculate the buffer ratio, and explain oxygen transport and carbon monoxide toxicity in terms of ligand substitution in haemoglobin.

Practise this question

Question

Question 17 in two parts. Part (a) asks to explain, in terms of equilibrium, how the carbonic acid-hydrogencarbonate buffer maintains blood pH of 7.40, given pKa = 6.38, and to calculate the [HCO3-]:[H2CO3] ratio, worth 6 marks. Part (b) asks to explain using ligand substitutions how haemoglobin transports oxygen and why carbon monoxide is toxic, worth 3 marks.
Question text

17 Healthy human blood needs to be maintained at a pH of 7.40 for the body to function normally.

(a)* Carbonic acid, H CO , is a weak acid which, together with hydrogencarbonate ions, HCO −,

23 3

acts as a buffer to maintain the pH of blood.

The pKa value for the dissociation of carbonic acid is 6.38.

Explain, in terms of equilibrium, how the carbonic acid–hydrogencarbonate mixture acts as a

buffer in the control of blood pH, and calculate the [HCO −] : [H CO ] ratio in healthy blood.

32 3

[6]

Additional answer space if required

(b) Red blood cells contain haemoglobin.

Explain using ligand substitutions:

• how haemoglobin transports oxygen around the body

• why carbon monoxide is toxic.

… [3]

Mark scheme

Show the mark scheme Mark scheme for question 17 showing Level 1 to Level 3 descriptors for part (a) covering equilibrium explanation, buffer action, and calculation of the ratio, alongside indicative points. Part (b) is marked out of 3 points for coordinate bonding of oxygen, ligand substitution of oxygen by water/carbon dioxide/carbon monoxide, and stronger binding of carbon monoxide.

AO

Question Answer Marks Guidance

element

17 (a) Please refer to the marking instructions on page 4 of this 6 1.1 ×2 Indicative scientific points may include:

mark scheme for guidance on how to mark this question. 1.2 ×2 (State symbols not required in equations)

3.1 ×1

Level 3 (5–6 mark) 3.2 ×1 Equilibrium and equilibrium shifts

Detailed explanation of equilibrium, the action of the buffer • H CO (aq) ⇌ H+(aq) + HCO –(aq)

23 3

and correct calculation of [HCO –] : [H CO ] ratio. +

32 3 • Addition of H causes ⇌ to shift to left

• Addition of OH– causes ⇌ to shift to right

There is a well-developed line of reasoning which is clear

and logically structured. The information presented is Action of buffer

relevant and substantiated. • Increase in H+ / addition of acid leads to:

H+(aq) + HCO –(aq) → H CO (aq)

Level 2 (3–4 marks) 3 2 3

OR HCO – reacts with added acid

Detailed explanation of equilibrium and the action of the buffer. 3

OR • Increase in OH– / addition of alkali leads to:

Detailed explanation of equilibrium and correct calculation of H+(aq) + OH–(aq) → H O(l)

– 2

[HCO3 ] : [H2CO3] ratio.

OR OR

Detailed explanation of the action of the buffer and correct H CO (aq) + OH–(aq) → HCO –(aq) + H O(l)

23 3 2

–

calculation of [HCO3 ] : [H2CO3] ratio. OR

OR

Partial explanations of equilibrium, and the action of the buffer H2CO3 reacts with added alkali

–

and attempt calculation of [HCO3 ] : [H2CO3] ratio. –

Calculation of [HCO3 ] : [H2CO3] ratio

• K = 10–6.38 OR 4.17 × 10–7 (mol dm–3)

There is a line of reasoning presented with some structure. a

• [H+] = 10–7.40 OR 3.98 x 10–8 (mol dm–3)

The information presented is relevant and supported by

some evidence. [HCO –] 4.17 x 10–7

• OR –8

[H2CO3] 3.98 x 10

Level 1 (1–2 marks) • ratio = 10.47(:1) OR 10.48(:1)

Detailed explanation of equilibrium.

OR ALLOW 10.5 OR 10(:1) (after working shown)

–

Correct calculation of [HCO3 ] : [H2CO3] ratio. –7

OR 4.2 x 10

ALLOW 4.0 x 10–8

Detailed explanation of the action of the buffer.

OR

Partial explanations of equilibrium and the action of the buffer.’ And ratio = 10.5 OR 11 (after working shown)

H432/01 Mark Schemes June 2019

10 AO

element

OR

Partial explanation of equilibrium and attempt at calculation of

– –

[HCO3 ] : [H2CO3] ratio.’ [H2CO3] 3.98 x 10

OR ALLOW [HCO –] OR 4.17 x 10–7

Partial explanation of the action of the buffer and attempt at

–

calculation of [HCO3 ] : [H2CO3] ratio.

There is an attempt at a logical structure with a line of And ratio = 1 : 0.095 ..

reasoning. The information is in the most part relevant.

0 marks

No response or no response worthy of credit.

17 (b) Coordinate bond mark 3 1.1 ×2 ALLOW names or symbols of ligands

O (coordinately or datively) bonds with Fe2+/Fe(II)/Fe/Iron ALLOW H O/CO/CO (coordinately or datively)

22 2

bonds with Fe2+/Fe(II)/Fe/Iron

ALLOW oxygen donates electron pair to OR

binds with Fe2+/Fe(II)/Fe/Iron

DO NOT ALLOW Fe3+

Ligand substitution mark ALLOW other words for replaced

(When required) O2 is replaced by H2O OR CO2

OR O2 is replaced by CO

OR H2O OR CO2 is replaced by O2

Ligand strength mark 2.1 ×1 ALLOW Kstab for CO (much) higher (than for O2)

CO forms strong(er) bonds (than O2) ALLOW CO bonds irreversibly OR CO is a

strong(er) ligand

IGNORE affinity

Total 9

How to answer it

OCR A-Level Chemistry: Blood Buffers & Ligand Substitution

What this question tests

This synoptic assessment evaluates your understanding of equilibria in weak acid-base buffer systems, calculating hydrogen ion concentrations and component ratios from pKa and pH values, and applying transition metal coordination chemistry (specifically ligand exchange and dative covalent bonding) to biological systems like haemoglobin and carbon monoxide toxicity.

Question 1 (a) - Total: [6] Marks

Carbonic Acid - Hydrogencarbonate Blood Buffer System

💡 Key Knowledge

  • Equilibrium establishment: H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)
  • Le Chatelier's Principle: Adding H⁺ shifts equilibrium left; adding OH⁻ reacts with H⁺ (or H₂CO₃) shifting equilibrium right.
  • Buffer Action: Hydrogencarbonate ions remove added H⁺; carbonic acid neutralises added OH⁻.

✅ Correct Answers & Mark Scheme

  • Equilibrium equation & shifts: Clearly state the equilibrium and explain both acid/alkali addition responses.
  • Equations for addition: H⁺ + HCO₃⁻ → H₂CO₃ and H⁺ + OH⁻ → H₂O (or H₂CO₃ + OH⁻ → HCO₃⁻ + H₂O ).
  • Ratio calculation result: 10.5 : 1 or 11 : 1 (or inverse 1 : 0.095 ).

📐 Calculation Steps

  1. Find K_a from pKa: K_a = 10⁻ᵖḰᵃ = 10⁻⁶·³⁸ = 4.17 × 10⁻⁷ mol dm⁻³
  2. Find [H⁺] from pH: [H⁺] = 10⁻ᵖᴴ = 10⁻⁷·⁴⁰ = 3.98 × 10⁻⁸ mol dm⁻³
  3. Rearrange K_a = ([H⁺][HCO₃⁻]) / [H₂CO₃] to find the ratio: [HCO₃⁻] / [H₂CO₃] = K_a / [H⁺]
  4. Calculate: (4.17 × 10⁻⁷) / (3.98 × 10⁻⁸) = 10.48... rounds to 10.5 : 1 (or 11 : 1).

❌ Common Errors & Traps

  • Forgetting to convert pKa into K_a using inverse log ( 10⁻ᵖḰᵃ ).
  • Inverting the ratio fraction ( [H₂CO₃] / [HCO₃⁻] instead of [HCO₃⁻] / [H₂CO₃] ).
  • Failing to explicitly link the shift in equilibrium to Le Chatelier's principle when explaining buffer action.
Examiner Commentary (Leveled Response): Part (a) is a Level of Response (Levels 1–3) question worth 6 marks. Top-level responses (Level 3) seamlessly integrated the equilibrium equations, clear descriptions of how both added acid and alkali are scavenged, and executed the multi-step calculation with correct significant figures and ratio formatting.
Question 1 (b) - Total: [3] Marks

Haemoglobin, Ligand Substitution, and Carbon Monoxide Toxicity

💡 Key Knowledge

  • Coordinate Bonding: Oxygen forms dative covalent (coordinate) bonds with Fe²⁺ in haemoglobin using lone pairs on oxygen.
  • Ligand Substitution: Reversible replacement of ligands in coordination complexes (e.g., O₂ replaced by H₂O or CO ).
  • Ligand Strength: Carbon monoxide forms significantly stronger dative bonds with Fe²⁺ than oxygen does, making the substitution virtually irreversible.

✅ Correct Answers & Mark Scheme

  • Mark 1 (Coordinate bond): O₂ coordinately/datively bonds to Fe²⁺ (Iron(II)). Note: Fe³⁺ is not allowed!
  • Mark 2 (Ligand substitution): O₂ is replaced by H₂O or CO (or vice versa).
  • Mark 3 (Ligand strength): CO forms stronger bonds than O₂ (higher stability constant / irreversible binding).

🧠 Exam Technique

Be extremely precise with oxidation states and terminology. Always refer to iron as Fe²⁺ or Iron(II). Avoid colloquialisms like "oxygen attaches"—use proper coordination terminology such as "coordinate bond", "dative covalent bond", and "ligand substitution".

❌ Common Errors & Traps

  • Writing Fe³⁺ instead of Fe²⁺ , which instantly loses the first mark.
  • Describing carbon monoxide poisoning as simply "blocking" sites without explaining that it undergoes a strong ligand substitution reaction.
  • Omitting the word "dative" or "coordinate" when describing how oxygen binds to the metal center.
Examiner Commentary: Candidates frequently lost marks here by using imprecise biological descriptions rather than precise chemical terminology. To secure full marks, students must explicitly reference coordinate/dative bonding, ligand exchange, and the relative bond strengths of CO versus O₂.

Topics

Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH · 5.3 Transition elements

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.