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S4 Chemistry Bonding, Paper 3: formula masses, electron arrangements and structure counting
S4 Chemistry Bonding, Paper 3: formula masses, electron arrangements and structure counting
Ten graded items linking bond counting to formula mass, atom ratios and the physical properties of structures.
Focus Formula mass adds relative atomic masses including water of crystallisation; atom ratios follow from counting both ends of each bond.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry: Bonding and Structure |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationFind the formula mass of sodium chloride, with Na as 23.0 and Cl as 35.5.
- FoundationHow many pairs of electrons are shared in a carbon dioxide molecule?
- FoundationFind the ionic charge product in magnesium oxide and compare the electrostatic attraction with that in sodium chloride.
- FoundationExplain why magnesium melts at a higher temperature than sodium, using the metallic structure.
- IntermediateFind the molar mass of magnesium chloride, with Mg as 24.3 and Cl as 35.5.
- IntermediateWrite the electron arrangement of the oxide ion, O2-, by shells.
- IntermediateHow many covalent bonds are there in a hydrogen peroxide molecule, H2O2?
- ChallengeIn silicon dioxide each silicon bonds to four oxygen atoms and each oxygen to two silicon atoms. Find the ratio of silicon to oxygen atoms.
- ChallengeFind the molar mass of hydrated copper sulphate, CuSO4.5H2O, with Cu as 63.5, S as 32.0, O as 16.0 and H as 1.0.
- Error redoA student says an ionic solid conducts because it already contains ions. Give the correct explanation.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 58.5 | The formula mass adds the relative atomic masses: 23.0 plus 35.5 gives 58.5. |
| 2 | Four pairs, eight electrons | Each carbon-oxygen link is a double bond with two shared pairs, four pairs in total. |
| 3 | 2 x 2 = 4, four times that of NaCl | The charge product is four rather than one, so the attraction in the lattice is four times as strong. |
| 4 | Magnesium ions carry two charges and release two delocalised electrons, giving stronger attraction | Magnesium ions carry two charges and release two delocalised electrons each, so the attraction is stronger than in sodium. |
| 5 | 95.3 g mol^-1 | 24.3 plus twice 35.5 gives 24.3 plus 71.0, which is 95.3. |
| 6 | 2, 8 | Oxygen has eight electrons and gains two, giving ten in the arrangement of neon. |
| 7 | Three single bonds | There is one bond between the two oxygen atoms and one to each hydrogen, three in all. |
| 8 | 1 to 2, that is SiO2 | Counting both ends of every bond, 4n silicon bonds equal 2m oxygen bonds, so oxygen is twice silicon. |
| 9 | 249.5 g mol^-1 | 63.5 plus 32.0 plus 64.0 plus 90.0 gives 249.5, of which the water contributes 90.0. |
| 10 | The ions are fixed in the lattice and cannot move; conduction needs molten or dissolved ions | In the solid the ions are locked in the lattice and cannot move, so it does not conduct; melting or dissolving frees them. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.
