OCR A-Level Chemistry AS Depth in chemistry (02), June 2024: Question 1

✅ Mark Scheme Model Answer

• The ability/tendency of an atom to attract electrons [1]
• in a covalent bond (or shared pair) [1]

❌ Common Examiner Traps

• Writing "element" instead of atom loses Mark 1.
• Stating "attract an electron" implies electron affinity (forming an ion), which loses Mark 1.
• Omitting "in a covalent bond" or "shared pair" forfeits Mark 2 completely.

✅ What Full-Mark Diagrams Must Show

🧠 Top Exam Technique

• Always draw both lone pairs on the oxygen atom as distinct lobes or electron pairs. The mark scheme strictly penalises drawing >2 lone pairs on oxygen!
• Ensure the dashed line terminates directly on the lone pair, not ambiguously on the oxygen nucleus or bond.
• Keep the O—H···O alignment roughly linear (180°) around the hydrogen atom.

✅ Model Answer

The dipoles do not cancel out / the molecule has an overall permanent dipole (because it is non-linear / unsymmetrical) [1].

❌ Common Misconceptions

• "Because the O-H bonds are polar." — Incorrect on its own! CO₂ has polar bonds but is non-polar because it is symmetrical.
• Saying "charges do not cancel" is ignored; use the term dipoles.

✅ Model Answer

In ice, molecules are held apart in an open lattice structure by hydrogen bonds [1].

❌ Forbidden Phrasing

• DO NOT write "ice traps air" or "gaps contain air". There is no air inside the crystal lattice!
• DO NOT write "atoms are held apart". You must refer specifically to molecules.

✅ Mark Scheme Points

• Ammonia forms weaker hydrogen bonds (or fewer hydrogen bonds per molecule) [1]
• Because nitrogen is less electronegative than oxygen OR nitrogen has only one lone pair whereas oxygen has two lone pairs [1]

❌ Critical Misconception

Never state that covalent bonds break upon melting. Stating "N-H covalent bonds are weaker than O-H bonds" immediately scores 0 for that point. Melting overcomes intermolecular hydrogen bonds, not intramolecular covalent bonds!

✅ Diagram Description

🧠 Examiner Checklist

• Total electrons around N must be exactly 8 (an octet).
• Ensure no extra unbonded lone pairs remain on N (it used its lone pair to bond to H⁺).
• Each H atom must only have 2 shared electrons and no extra outer electrons.

✅ Model Answer

• Reagent & Condition: Warm/heat with aqueous sodium hydroxide (NaOH / OH⁻) [1]
• Observation: Pungent gas evolved that turns moist red litmus paper (or universal indicator / pH paper) blue [1]

❌ Common Errors

• Adding ammonia or ammonium hydroxide as the reagent (contradictory and scores 0).
• Forgetting to state warm / heat. Without heat, NH₃ gas will not be displaced efficiently out of solution.
• Stating "litmus turns red" (ammonia is alkaline, it turns red litmus blue!).

📐 Step-by-Step Calculation

1. Calculate Molar Mass of Hydrated Salt:
   M(ZnSO₄•7H₂O) = 65.4 + 32.1 + (4 × 16.0) + 7 × 18.0 = 287.5 g mol⁻¹ [1]
2. Calculate Amount (moles) of ZnSO₄•7H₂O:
   n = mass / M = 11.50 / 287.5 = 0.0400 mol [1]
3. Calculate Mass of Anhydrous ZnSO₄:
   Stoichiometric ratio is 1:1, so n(ZnSO₄) = 0.0400 mol .
   M(ZnSO₄) = 65.4 + 32.1 + (4 × 16.0) = 161.5 g mol⁻¹
   mass = 0.0400 × 161.5 = 6.46 g [1]

🧠 Alternative Method & Checks

Alternative via mass of water:

• n(H₂O) = 0.0400 × 7 = 0.280 mol
• mass(H₂O) = 0.280 × 18.0 = 5.04 g
• mass(ZnSO₄) = 11.50 - 5.04 = 6.46 g

Significant Figures: The answer should be quoted to at least 2 or 3 sig figs (6.46 g or 6.5 g accepted). Full 3 marks are awarded automatically if 6.46 g is on the answer line!