H₂O CO₂
NaCl NH₃
CH₄ C₆H₁₂O₆
A-LEVEL CHEMISTRY
OCR Chemistry A-Level
Questions That Seem Easy
But Are Tricky
— and How to Answer Them
Master the tricky topics | Step-by-step worked answers | Exam-ready strategies
30+ 100+ 10
Topics Q & A Pairs Chapters
For Students, By Chemistry Experts | Study Smart. Score Higher.
,OCR Chemistry A-Level | Tricky Questions & How to Answer Them Page 1
Table of Contents
Chapter 1 Atomic Structure & Electron Configuration — p.4
Chapter 2 Bonding, Structure & Properties — p.7
Chapter 3 The Mole, Formulae & Equations — p.10
Chapter 4 Acids, Bases & pH — p.13
Chapter 5 Redox Reactions — p.16
Chapter 6 Organic Chemistry: Functional Groups — p.19
Chapter 7 Energetics & Thermodynamics — p.22
Chapter 8 Kinetics & Equilibrium — p.25
Chapter 9 Electrochemistry & Electrolysis — p.28
Chapter 10 Periodic Trends & Transition Metals — p.31
Each chapter covers questions that look straightforward but contain common traps. Read the
question carefully, then check your answer against the model answer. The explanations tell
you exactly why the wrong answer is wrong — not just what the right answer is.
Study Smart. Score Higher.
, OCR Chemistry A-Level | Tricky Questions & How to Answer Them Page 2
Chapter 1 — Atomic Structure & Electron Configuration
Chapter 1: Atomic Structure & Electron
Configuration
Atomic structure forms the foundation of all chemistry. The tricky part is that questions often
test the difference between ions and atoms, isotopes and ions, or ask you to work out
electron arrangements for elements you don't recognise. Let's look at the types of questions
that catch people out.
Q An atom of element X has a mass number of 39 and 20 neutrons. What is the
atomic number of X, and what is the symbol for this element?
1
⚠ Trap Many students confuse mass number with atomic number. Mass
number = protons + neutrons. Atomic number = protons only. The
name and symbol of an element depends on its proton number
alone.
ANSW Mass number = protons + neutrons, so protons = 39 − 20 = 19. Atomic
ER number 19 is potassium (K). The 20 neutrons tell you it is an isotope of
potassium — specifically ⁃K-39.
Q Write the full electron configuration of a Ca²⁺ ion.
2
⚠ Trap Students often write the configuration of neutral calcium (Ca,
Z=20) and forget that Ca²⁺ has lost 2 electrons from the
outermost shell.
ANSW Neutral Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² (20 electrons). Ca²⁺ loses 2 electrons: 1s²
ER 2s² 2p⁶ 3s² 3p⁶ (18 electrons). This is the same configuration as Argon — a
noble gas.
Q Two isotopes of chlorine exist: ³⁵Cl and ³⁷Cl in a 3:1 ratio. Calculate the relative
atomic mass of chlorine.
3
Key Formula Relative Atomic Mass = (mass × abundance) summed for all
isotopes, divided by total abundance. Use the ratio as
percentages: 75% and 25%.
Study Smart. Score Higher.
NaCl NH₃
CH₄ C₆H₁₂O₆
A-LEVEL CHEMISTRY
OCR Chemistry A-Level
Questions That Seem Easy
But Are Tricky
— and How to Answer Them
Master the tricky topics | Step-by-step worked answers | Exam-ready strategies
30+ 100+ 10
Topics Q & A Pairs Chapters
For Students, By Chemistry Experts | Study Smart. Score Higher.
,OCR Chemistry A-Level | Tricky Questions & How to Answer Them Page 1
Table of Contents
Chapter 1 Atomic Structure & Electron Configuration — p.4
Chapter 2 Bonding, Structure & Properties — p.7
Chapter 3 The Mole, Formulae & Equations — p.10
Chapter 4 Acids, Bases & pH — p.13
Chapter 5 Redox Reactions — p.16
Chapter 6 Organic Chemistry: Functional Groups — p.19
Chapter 7 Energetics & Thermodynamics — p.22
Chapter 8 Kinetics & Equilibrium — p.25
Chapter 9 Electrochemistry & Electrolysis — p.28
Chapter 10 Periodic Trends & Transition Metals — p.31
Each chapter covers questions that look straightforward but contain common traps. Read the
question carefully, then check your answer against the model answer. The explanations tell
you exactly why the wrong answer is wrong — not just what the right answer is.
Study Smart. Score Higher.
, OCR Chemistry A-Level | Tricky Questions & How to Answer Them Page 2
Chapter 1 — Atomic Structure & Electron Configuration
Chapter 1: Atomic Structure & Electron
Configuration
Atomic structure forms the foundation of all chemistry. The tricky part is that questions often
test the difference between ions and atoms, isotopes and ions, or ask you to work out
electron arrangements for elements you don't recognise. Let's look at the types of questions
that catch people out.
Q An atom of element X has a mass number of 39 and 20 neutrons. What is the
atomic number of X, and what is the symbol for this element?
1
⚠ Trap Many students confuse mass number with atomic number. Mass
number = protons + neutrons. Atomic number = protons only. The
name and symbol of an element depends on its proton number
alone.
ANSW Mass number = protons + neutrons, so protons = 39 − 20 = 19. Atomic
ER number 19 is potassium (K). The 20 neutrons tell you it is an isotope of
potassium — specifically ⁃K-39.
Q Write the full electron configuration of a Ca²⁺ ion.
2
⚠ Trap Students often write the configuration of neutral calcium (Ca,
Z=20) and forget that Ca²⁺ has lost 2 electrons from the
outermost shell.
ANSW Neutral Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² (20 electrons). Ca²⁺ loses 2 electrons: 1s²
ER 2s² 2p⁶ 3s² 3p⁶ (18 electrons). This is the same configuration as Argon — a
noble gas.
Q Two isotopes of chlorine exist: ³⁵Cl and ³⁷Cl in a 3:1 ratio. Calculate the relative
atomic mass of chlorine.
3
Key Formula Relative Atomic Mass = (mass × abundance) summed for all
isotopes, divided by total abundance. Use the ratio as
percentages: 75% and 25%.
Study Smart. Score Higher.
