Edexcel A-Level Chemistry Paper 1, June 2025: Question 5

✅ Correct Answer

C: Acid = H₃O⁺, Conjugate base = H₂O

💡 Key Knowledge

A conjugate pair consists of two species that transform into one another by the gain or loss of one single proton (H⁺):

Acid ⇌ Base + H⁺

• Pair 1: HNO₃ (acid) / NO₃⁻ (conjugate base)
• Pair 2: H₃O⁺ (acid) / H₂O (conjugate base)

❌ Common Errors & Distractor Traps

• Option A (HNO₃ / H₂O): These are two reactants from completely different pairs; they do not interconvert via a single H⁺.
• Option B (NO₃⁻ / HNO₃): The columns are swapped. NO₃⁻ is a proton acceptor (base), not an acid.
• Option D (H₂O / H₃O⁺): The columns are reversed. H₂O is the base and H₃O⁺ is the conjugate acid.

🧠 Exam Technique

Always write out the forward and backward transfers:

Look at H₂O + H⁺ ⇌ H₃O⁺ . The species with more hydrogens is always the acid of the pair, while the species with one fewer hydrogen is the conjugate base.

✅ Correct Answer

pH = −log₁₀[H⁺(aq)]

or

pH = −log₁₀[H₃O⁺(aq)]

❌ Common Examiner Traps (Strict Penalties)

• Missing Square Brackets: Writing −log₁₀(H⁺) or −log₁₀ H⁺ scores 0 marks. In chemistry, square brackets specifically denote concentration in mol dm⁻³.
• Writing "conc": Writing −log conc H⁺ is not credited.
• Omitting the Minus Sign: Writing log[H⁺] instead of −log[H⁺] is an immediate loss of the mark.

🧠 Exam Technique

Even though the question says "State the expression...", always include pH = in front to form a complete mathematical equation. Use unambiguous square brackets [ ] around the chemical formula.

✅ Correct Answer

B: 3.16 × 10⁻⁵ (mol dm⁻³)

📐 Step-by-Step Calculation

Given: pH = 9.50, T = 298 K (K_w = 1.00 × 10⁻¹⁴ mol² dm⁻⁶)

1. Method 1 (via pOH):
   pH + pOH = 14.00
   pOH = 14.00 − 9.50 = 4.50
   [OH⁻] = 10^−pOH = 10^−4.50 = 3.16 × 10⁻⁵ mol dm⁻³
2. Method 2 (via [H⁺] & K_w):
   [H⁺] = 10^−pH = 10^−9.50 = 3.162 × 10⁻¹⁰ mol dm⁻³
   [OH⁻] = K_w / [H⁺] = (1.00 × 10⁻¹⁴) / (3.162 × 10⁻¹⁰) = 3.16 × 10⁻⁵ mol dm⁻³

❌ The "Ba(OH)₂" Stoichiometry Trap

• Why Option C (6.32 × 10⁻⁵) is WRONG: Students see Ba(OH)₂ and think: "There are 2 OH⁻ ions, so I must multiply by 2!"
  The Trap: The pH is measured directly from the solution. Therefore, the calculated [OH⁻] is already the total concentration of OH⁻ present in that solution! You would only multiply by 2 if you were given the concentration of Ba(OH)₂ solid dissolved and asked to find [OH⁻].
• Why Option A (3.16 × 10⁻¹⁰) is WRONG: This is [H⁺], not [OH⁻].
• Why Option D (3.16 × 10⁴) is WRONG: Power sign error: calculated using 10^+4.5 instead of 10^−4.5.

🧠 Top Tip: Measured vs. Theoretical

Ask yourself: "Where did the number come from?"

• If starting from pH ➔ It's an experimental measurement of the actual equilibrium mixture. Never multiply or divide by stoichiometric coefficients to get [H⁺] or [OH⁻].
• If starting from weighed mass/concentration of a base (e.g. 0.1 mol dm⁻³ Ba(OH)₂) ➔ Then you multiply by 2 to find [OH⁻].