Cambridge International A-Level Chemistry 9701: Frequent mistakes

Cambridge Principal Examiner Reports for Chemistry 9701 identify recurring weaknesses in molecular structure, atomic structure, calculations and question reading. Mean scores on multiple-choice papers are often low (e.g. ~20/40).

Molecular geometry and structure

Bond angles and shapes

Candidates struggle with bond angles and molecular shapes. Many evaluate angles from diagrams rather than calculating actual angles (e.g. 109.5° for tetrahedral, 120° for trigonal planar).

Fix: Learn VSEPR. Tetrahedral: 109.5°. Trigonal planar: 120°. Linear: 180°. Calculate; don’t guess from diagram.

”All bonds remain in the places shown”

Candidates miss critical phrases like “all bonds remain in the places shown” that change the correct answer. Ignoring such constraints leads to conceptual errors.

Fix: Read every word. Constraints limit valid answers.

Atomic structure

Orbitals vs. subshells

Only ~5% correctly answered a question on 3p orbitals vs. 3p subshell. Confusion: one 3p orbital holds 2 electrons; 3p subshell has 3 orbitals (6 electrons).

Fix: Orbital = max 2 electrons. Subshell 3p = 3 orbitals = max 6 electrons.

Thermodynamics and enthalpy

Enthalpy of hydration

Many incorrectly believe enthalpy change of hydration can be positive. Bond formation (hydration) always releases energy— hydration is exothermic.

Fix: Hydration = bonds forming between ions and water. Bond formation = exothermic. ΔH_hydration < 0.

Calculations and stoichiometry

Limiting reagent

Frequent struggle with limiting reagent problems. Failing to identify which reactant limits product yield; wrong stoichiometry.

Fix: Find moles of each. Compare with ratio. Limiting reactant determines max product.

Isomer counting

Missing isomers when counting structural possibilities. E.g. missing one of five trichloropropane isomers—incomplete systematic enumeration.

Fix: Be systematic. Consider position of each substituent. Draw all possibilities.

Time management

Many candidates fail to finish papers within the time allowed. Time allocation by marks is essential.

Fix: Allocate time by marks. Don’t over-write on early questions.

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Based on Cambridge International A-level Chemistry 9701 Principal Examiner Reports (2016–2024).