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CAIE A-Level Chemistry 24.1.3 Electrolysis Calculations

Practise calculating charge, current, time and the mass or gas volume liberated using electron stoichiometry.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • calculate charge in coulombs from Q=It after converting the electrolysis time to seconds
  • convert charge to moles of electrons using n(e−)=Q/F and use the balanced half-equation ratio
  • convert product moles to mass or gas volume, or reverse the chain to determine current or time

24.1.3—Calculate question 1

[Maximum number: 3]

Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry.

A student electrolyses a solution of KI(aq) for 8 minutes using a direct current. The half-equation for the reaction that occurs at the anode is given.

2I−(aq)→I2(aq)+2e−2 \mathrm{I}^{-}(\mathrm{aq}) \rightarrow \mathrm{I}_{2}(\mathrm{aq})+2 \mathrm{e}^{-}

After the electrolysis, the I2(aq)I_{2}(a q) produced requires 21.35 cm321.35 \mathrm{~cm}^{3} of 0.100moldm−3Na2 S2O3(aq)0.100 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq}) to react completely.

I2(aq)+2Na2 S2O3(aq)→2NaI(aq)+Na2 S4O6(aq)\mathrm{I}_{2}(\mathrm{aq})+2 \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq}) \rightarrow 2 \mathrm{NaI}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{~S}_{4} \mathrm{O}_{6}(\mathrm{aq})

Calculate the average current used in 8 minutes during the electrolysis.
current = A

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