Calculate charge passed during electrolysis
Use current and time to determine total charge and then convert charge to moles of electrons.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 24. Treat all tabulated electrode potentials as reduction potentials; state conditions, phases, signs and units. Written practice is self-checked
Before you begin
- Use AS redox half-equations, oxidation states, mole ratios, current and charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which formula relates charge, current and time?
What you will learn
- Use Q = It
- Convert time to seconds and current to amperes
- Convert charge into moles of electrons using F
Current is charge transferred each second
Charge Q in coulombs is calculated from current I in amperes and time t in seconds: Q = It. Since 1 A = 1 C s⁻¹, the units multiply to coulombs.
If the current varies with time, use the appropriate measured or average current for the stated interval. Do not use minutes directly unless you first convert them to seconds.
Convert charge to an electron amount
One mole of electrons carries F coulombs, so n(e⁻) = Q/F. This tells you the amount of electrons transferred through the external circuit; use the electrode half-equation to connect it to the amount of product.
The electron charge has a direction in a circuit, but Q and n(e⁻) in these electrolysis calculations use positive magnitudes. Assign the electrode reaction separately.
Pause and explain
What is the unit of Q when I is in amperes and t is in seconds?
Worked example
A current of 2.0 A flows for 15 minutes. Calculate Q and n(e⁻), using F = 9.65 × 10⁴ C mol⁻¹.
Show the worked solution
- Convert 15 minutes to 900 seconds.
- Calculate Q = It = 2.0 × 900.
- Divide Q by F.
Answer Q = 1800 C; n(e⁻) = 1800/(9.65 × 10⁴) = 0.0187 mol.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A 0.50 A current flows for 20 minutes. Calculate Q and moles of electrons transferred.
Give me a hint
Convert to seconds, calculate It, then divide by F = 9.65 × 10⁴ C mol⁻¹.
Compare my reasoning
- 20 minutes = 1200 s.
- Q = 0.50 × 1200.
- n(e⁻) = 600/(9.65 × 10⁴).
Q = 600 C; n(e⁻) = 0.00622 mol.
Look for these in your work
- Time converted
- Charge calculated
- Electron amount calculated
A cell passes 4.82 × 10³ C. Calculate the amount of electrons transferred using F = 9.65 × 10⁴ C mol⁻¹.
Give me a hint
Use n(e⁻) = Q/F.
Compare my reasoning
- Write n(e⁻) = Q/F.
- Substitute 4.82 × 10³ and 9.65 × 10⁴.
- Give mol.
n(e⁻) = 4.82 × 10³/(9.65 × 10⁴) = 0.0500 mol.
Look for these in your work
- Relation chosen
- Substitution correct
- Mole unit stated
A learner uses 30 minutes in Q = (0.80 A)(30) and reports 24 C. Explain the error and correct Q.
Give me a hint
Time must be in seconds for amperes to cancel to coulombs.
Compare my reasoning
- Convert 30 min to 1800 s.
- Multiply by 0.80 A.
- Check A·s gives C.
The time was not converted: 30 min = 1800 s. Q = 0.80 × 1800 = 1440 C.
Look for these in your work
- Time conversion identified
- Correct charge calculated
- Units checked
Common mistakes
- Using minutes without conversion
- Multiplying current by charge instead of time
- Reporting amperes as the charge unit
- Using the wrong conversion direction for Q/F
What can you explain now?
Which formula relates charge, current and time?
Compare with the explanation
Q = It
Current is charge transferred per second.
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Make it stick
How do current and time give charge, and how does Faraday's constant convert that charge to electron moles?
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