WASSCE Chemistry · Paper 1 style

Electrochemistry

One sample question with the full working. 14 more in the app, with instant feedback and repair sessions for the traps below.

A current of 2 A is passed through a solution containing Cu²⁺ ions for 965 s. Calculate the mass of metal deposited. [F = 96 500 C mol⁻¹; relative atomic mass = 64]

A1.28 g
B0.0107 g
C0.29 g
D0.64 g

Show the answer and working

Answer: D

Q = It = 2 × 965 = 1930 C, so moles of electrons = 1930/96 500 = 0.02; Cu²⁺ needs 2 e⁻ per atom, so moles of Cu = 0.01 and mass = 0.01 × 64 = 0.64 g.

Trap A Forgets the charge on the ion in Faraday calculations: Moles of metal = moles of electrons ÷ charge on the ion. Cu²⁺ + 2e⁻ → Cu: 2 F deposits 1 mol of copper.

Trap B Uses minutes instead of seconds: Q (coulombs) = I (amperes) × t (seconds). Convert minutes and hours to seconds before substituting.

Trap C Uses the atomic number instead of the relative atomic mass: Atomic number counts protons; relative atomic mass gives the mass of a mole. Mass calculations always use Ar (or Mr).

Traps in Electrochemistry

Forgets the charge on the ion in Faraday calculations
Moles of metal = moles of electrons ÷ charge on the ion. Cu²⁺ + 2e⁻ → Cu: 2 F deposits 1 mol of copper.
Uses minutes instead of seconds
Q (coulombs) = I (amperes) × t (seconds). Convert minutes and hours to seconds before substituting.
Uses the atomic number instead of the relative atomic mass
Atomic number counts protons; relative atomic mass gives the mass of a mole. Mass calculations always use Ar (or Mr).
Picks a related substance or process with a different role
Learn reagents and processes as one-line 'name → job' cards and contrast neighbours in a table (process | product | catalyst | conditions). Match the job in the stem, not just the topic.
Predicts wrong electrolysis products
Cations go to the CATHODE (reduction): less reactive metal or H₂. Anions go to the ANODE (oxidation): halogen if concentrated halide, else O₂ from OH⁻. Active anodes (Cu) dissolve.
Swaps anode and cathode products
Cations go to the Cathode and gain electrons (reduction): metals or H₂. Anions go to the Anode and lose electrons (oxidation): O₂, Cl₂, or an active anode dissolves. 'An Ox, Red Cat.'
Uses everyday or macroscopic thinking instead of particles
Ask what the particles, ions or electrons are doing. Properties of a substance come from its particles and bonds, not from how it looks or feels.
Over-generalises a rule beyond where it holds
Every rule in chemistry has conditions. Before applying one, check the conditions in the stem (electrode type, concentration, moles of gas on each side, metal position).
Properties of metals and compounds errors
Reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au. Amphoteric oxides/hydroxides: Al, Zn, Pb. Rusting needs BOTH water and oxygen; prevented by painting, galvanising (zinc), sacrificial protection.
Reverses a trend, direction or effect
State the rule with its direction before looking at the options (down group 1 reactivity INCREASES; exothermic forward reaction: heating shifts LEFT). An option that is the mirror image of the rule is the trap.
Mixes up a pair of similar chemical terms
Keep a list of confusable pairs with one defining feature each and a hook. Underline the key word in the stem and check it against the definition, not the sound of the name.
Takes one factor as the whole answer
Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.

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