EDEXCEL A-LEVEL CHEMISTRY
Questions That Seem
Easy But Tricky
...and How to Answer Them
30 Topics Real Exam Traps Step-by-Step Solutions Visual Guides
Master the questions that catch every student off guard
★ The Ultimate Exam Preparation Guide ★
, Table of Contents
Navigate Your Way to Exam Success
Your complete guide to the questions that
look simple — until you get them wrong.
1. Moles, Masses and the Mole Concept ............... 4
2. Atomic Structure – The Sneaky Details ............ 5
3. Bonding and Structure Traps ...................... 6
4. Energetics – Signs and State Symbols ............. 8
5. Equilibrium – Le Chatelier Traps ................. 10
6. Acids, Bases and pH Calculations ................. 12
7. Redox – Oxidation State Traps .................... 14
8. Organic Chemistry – Naming Nightmares ............ 16
9. Reaction Kinetics – Rate Equation Traps .......... 18
10. Electrochemistry and Cell EMF ................... 20
11. Transition Metals – Colour and Catalysis ........ 22
12. Polymers and Green Chemistry .................... 24
13. Spectroscopy – IR, NMR, Mass Spec ............... 26
14. Titration Calculations and Errors ............... 28
Quick-Reference Cheat Sheet ..................... 30
How to Use Each chapter starts with the key concept, shows you the most
common mistakes, then walks through the correct approach step
by step. Use the Q&A blocks to test yourself before reading the
answer.
Edexcel A-Level Chemistry | Questions That Seem Easy But Tricky
, EDEXCEL A-LEVEL CHEMISTRY Tricky Questions Guide
CHAPTER 1
Moles, Masses and the Mole Concept
Mole calculations are everywhere in A-level chemistry. They look straightforward, but
small mistakes — especially with units and formulae — cost marks.
The Core Formula
Everything starts with: n = m / M where n = moles, m = mass in grams, M = molar mass in
g/mol. Simple — but the traps hide in the details.
Q 0.50 mol of CaCO3 is heated. What mass of CO2 is produced?
■ COMMON TRAP Students forget to check the 1:1 mole ratio and often use the
molar mass of CaCO3 instead of CO2.
✔ ANSWER CaCO3 → CaO + CO2. Mole ratio 1:1, so 0.50 mol CO2
produced. M(CO2) = 44 g/mol. Mass = 0.50 × 44 = 22 g.
Pro Tip: Always write the balanced equation first. The mole ratio is your bridge between species.
Q Calculate the number of atoms in 9.0 g of water.
■ COMMON TRAP Many students find the number of molecules and forget to
multiply by 3 (2H + 1O per molecule).
✔ ANSWER M(H2O) = 18 g/mol. Moles = 9.0/18 = 0.50 mol. Molecules =
0.50 × 6.02×1023 = 3.01×1023. Atoms = 3.01×1023 × 3 =
9.03×1023 atoms.
Pro Tip: Atoms ≠ molecules. Water has 3 atoms per molecule.
Percentage Yield and Atom Economy
Term Formula Common Error
% Yield (actual/theoretical) × 100 Using mass instead of moles
Atom Economy (useful M<sub>r</sub> / total M<sub>r</sub>)
Including ×by-products
100 in numerator
Mole Fraction n(X) / n(total) Forgetting to include ALL gases
Q The atom economy of: CH4 + 2Cl2 → CH2Cl2 + 2HCl
■ COMMON TRAP Students often put both products in the numerator, or forget to
sum all Mr values on the right.
✔ ANSWER Desired product = CH2Cl2, Mr = 85. Total Mr of products = 85 +
2×36.5 = 158. Atom economy = (85/158) × 100 = 53.8%.
Pro Tip: Only the useful product goes on top.
Questions That Seem Easy But Tricky | Edexcel A-Level Chemistry | Page 3
Questions That Seem
Easy But Tricky
...and How to Answer Them
30 Topics Real Exam Traps Step-by-Step Solutions Visual Guides
Master the questions that catch every student off guard
★ The Ultimate Exam Preparation Guide ★
, Table of Contents
Navigate Your Way to Exam Success
Your complete guide to the questions that
look simple — until you get them wrong.
1. Moles, Masses and the Mole Concept ............... 4
2. Atomic Structure – The Sneaky Details ............ 5
3. Bonding and Structure Traps ...................... 6
4. Energetics – Signs and State Symbols ............. 8
5. Equilibrium – Le Chatelier Traps ................. 10
6. Acids, Bases and pH Calculations ................. 12
7. Redox – Oxidation State Traps .................... 14
8. Organic Chemistry – Naming Nightmares ............ 16
9. Reaction Kinetics – Rate Equation Traps .......... 18
10. Electrochemistry and Cell EMF ................... 20
11. Transition Metals – Colour and Catalysis ........ 22
12. Polymers and Green Chemistry .................... 24
13. Spectroscopy – IR, NMR, Mass Spec ............... 26
14. Titration Calculations and Errors ............... 28
Quick-Reference Cheat Sheet ..................... 30
How to Use Each chapter starts with the key concept, shows you the most
common mistakes, then walks through the correct approach step
by step. Use the Q&A blocks to test yourself before reading the
answer.
Edexcel A-Level Chemistry | Questions That Seem Easy But Tricky
, EDEXCEL A-LEVEL CHEMISTRY Tricky Questions Guide
CHAPTER 1
Moles, Masses and the Mole Concept
Mole calculations are everywhere in A-level chemistry. They look straightforward, but
small mistakes — especially with units and formulae — cost marks.
The Core Formula
Everything starts with: n = m / M where n = moles, m = mass in grams, M = molar mass in
g/mol. Simple — but the traps hide in the details.
Q 0.50 mol of CaCO3 is heated. What mass of CO2 is produced?
■ COMMON TRAP Students forget to check the 1:1 mole ratio and often use the
molar mass of CaCO3 instead of CO2.
✔ ANSWER CaCO3 → CaO + CO2. Mole ratio 1:1, so 0.50 mol CO2
produced. M(CO2) = 44 g/mol. Mass = 0.50 × 44 = 22 g.
Pro Tip: Always write the balanced equation first. The mole ratio is your bridge between species.
Q Calculate the number of atoms in 9.0 g of water.
■ COMMON TRAP Many students find the number of molecules and forget to
multiply by 3 (2H + 1O per molecule).
✔ ANSWER M(H2O) = 18 g/mol. Moles = 9.0/18 = 0.50 mol. Molecules =
0.50 × 6.02×1023 = 3.01×1023. Atoms = 3.01×1023 × 3 =
9.03×1023 atoms.
Pro Tip: Atoms ≠ molecules. Water has 3 atoms per molecule.
Percentage Yield and Atom Economy
Term Formula Common Error
% Yield (actual/theoretical) × 100 Using mass instead of moles
Atom Economy (useful M<sub>r</sub> / total M<sub>r</sub>)
Including ×by-products
100 in numerator
Mole Fraction n(X) / n(total) Forgetting to include ALL gases
Q The atom economy of: CH4 + 2Cl2 → CH2Cl2 + 2HCl
■ COMMON TRAP Students often put both products in the numerator, or forget to
sum all Mr values on the right.
✔ ANSWER Desired product = CH2Cl2, Mr = 85. Total Mr of products = 85 +
2×36.5 = 158. Atom economy = (85/158) × 100 = 53.8%.
Pro Tip: Only the useful product goes on top.
Questions That Seem Easy But Tricky | Edexcel A-Level Chemistry | Page 3
