How do I turn moles of electrons into a gas volume?

Read the half-equation for how many electrons give one gas molecule, then multiply by the molar volume of 24 dm^3.

What is the order for a time question?

Work back from the target mass to moles, then electrons, then charge, and divide by the current last.

S5 Chemistry Electrolysis, Paper 4: charge, gas volume and time

2026-11-02

Tutor Prep Guide · Primary English

Ten graded items from Q equals I t to metal masses, gas volumes and the time a current must flow.

Focus　Fix the units before computing charge; divide by the ion charge for metals, use two electrons per hydrogen molecule, and divide by current only at the end.

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: S5 Chemistry: Electrolysis
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationA current of 1.0 A flows for 100 s. Find the charge passed.
FoundationAt which electrode does reduction occur during electrolysis?
FoundationWhat material is normally used for the anode in electroplating?
FoundationWhich gas forms at the anode when dilute sulphuric acid is electrolysed?
IntermediateCopper sulphate solution is electrolysed at 0.50 A for 19 300 s. Find the mass of copper deposited, with Cu as 63.5.
IntermediateHow many dm^3 of hydrogen could be released at the cathode by 0.20 mol of electrons, taking the molar volume as 24 dm^3?
IntermediateWhat forms at the cathode when molten sodium chloride and aqueous sodium chloride are electrolysed?
ChallengeSilver nitrate solution is electrolysed at 3.0 A for 3 860 s. Find the mass of silver deposited, with Ag as 107.9.
ChallengeHow long must 2.0 A flow to deposit 1.00 g of copper, with Cu as 63.5 and F as 96 500?
Error redoA student says electrons travel through the electrolyte so the solution conducts. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	100 C	Charge is current times time, so 1.0 times 100 gives 100 coulombs.
2	The cathode	The cathode gains electrons, and gaining electrons is the definition of reduction.
3	The pure metal being plated	The anode dissolves to replace the metal ions in the bath, so the pure plating metal keeps the concentration steady.
4	Oxygen	Hydroxide ions discharge at the anode to give oxygen, while hydrogen forms at the cathode.
5	3.18 g	9 650 coulombs is 0.10 mol of electrons, and a divalent ion takes half of that, giving 3.18 grams.
6	2.4 dm^3	Two electrons give one hydrogen molecule, so 0.10 mol of gas forms and occupies 2.4 dm^3.
7	Sodium from the melt, hydrogen from the solution	In aqueous solution hydrogen ions discharge more readily than sodium ions, so hydrogen forms instead of sodium.
8	12.95 g	11 580 coulombs is 0.12 mol of electrons, and silver is monovalent so 0.12 mol deposits 12.95 grams.
9	About 1 520 s	1.00 gram of copper is 0.0157 mol, needing 0.0315 mol of electrons or 3 039 coulombs, which at 2.0 A takes about 1 520 seconds.
10	Electrons stay in the electrodes and wires; ions carry the current inside the solution	Electrons stay in the electrodes and the external circuit; inside the solution the current is carried by migrating ions.

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.