Question 1 Medium
Dilute sulfuric acid is electrolysed using inert electrodes. 18 cm³ of oxygen is collected. Calculate the volume of hydrogen collected.
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Answer: 36 cm³
- The ratio of hydrogen to oxygen is 2 : 1.
- 18 × 2 = 36.
Electrolysis of aqueous solutions: GCSE Combined Science Chemistry practice questions
A second set of questions on electrolysis of aqueous solutions. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
More questions
Question 2 Medium
Dilute sulfuric acid is electrolysed using inert electrodes. 50 cm³ of hydrogen is collected. Calculate the volume of oxygen collected.
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Answer: 25 cm³
- The ratio of hydrogen to oxygen is 2 : 1.
- 50 ÷ 2 = 25.
Question 3 Medium
A copper anode has a mass of 8.40 g before electrolysis and 8.05 g after. Calculate the mass lost.
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Answer: 0.35 g
- Mass lost = 8.40 - 8.05.
- Mass lost = 0.35 g.
Question 4 Medium
State what is left in the solution when brine is electrolysed, apart from water.
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Answer: sodium hydroxide solution
- Hydrogen and chlorine leave as gases.
- Na⁺ and OH⁻ ions remain, so sodium hydroxide solution is left.
Question 5 Hard
Zinc sulfate solution is electrolysed using inert electrodes. Name the product at the cathode and explain why.
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Answer: hydrogen, because zinc is more reactive than hydrogen
- Zinc is more reactive than hydrogen.
- So H⁺ is discharged and hydrogen forms.
Question 6 Hard
Explain why the blue colour of copper sulfate solution fades when it is electrolysed using inert electrodes.
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Answer: copper ions are removed from the solution as copper forms at the cathode
- Blue Cu²⁺ ions are discharged at the cathode.
- So fewer copper ions remain in the solution.
Question 7 Hard
Write a half equation for the formation of hydrogen at the cathode.
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Answer: 2H⁺ + 2e⁻ → H₂
- Two hydrogen ions make one H₂ molecule.
- Each gains one electron.
Question 8 Hard
Write a half equation for the formation of oxygen from hydroxide ions at the anode.
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Answer: 4OH⁻ → O₂ + 2H₂O + 4e⁻
- Four hydroxide ions lose four electrons.
- They make one O₂ molecule and two water molecules.
Question 9 Hard
Write a half equation for a copper anode dissolving in copper sulfate solution.
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Answer: Cu → Cu²⁺ + 2e⁻
- A copper atom loses two electrons.
- It becomes a Cu²⁺ ion.
Question 10 Hard
The cathode gains 0.64 g of copper. Calculate the moles of copper deposited. Ar of Cu = 63.5.
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Answer: 0.0101 mol
- Moles = mass ÷ Ar.
- 0.64 ÷ 63.5 = 0.0101.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
Which two ions does water add to every aqueous solution?
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H⁺ and OH⁻ ions.
Check 2
When is hydrogen formed at the cathode in electrolysis of a solution?
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When the metal in the salt is more reactive than hydrogen.
Check 3
When is the metal formed at the cathode in electrolysis of a solution?
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When the metal is less reactive than hydrogen, such as copper or silver.
Check 4
What forms at the anode if the solution contains a halide?
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The halogen: chlorine, bromine or iodine.
Check 5
What forms at the anode if there is no halide?
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Oxygen, from the hydroxide ions.
Check 6
Are sulfate and nitrate ions discharged at the anode?
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No. They stay in solution.
Check 7
What are the products of electrolysing dilute sulfuric acid?
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Hydrogen at the cathode and oxygen at the anode, in a 2 : 1 volume ratio.
Check 8
What are the products of electrolysing brine?
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Hydrogen at the cathode, chlorine at the anode and sodium hydroxide solution left behind.
Check 9
What are the products of electrolysing copper sulfate with inert electrodes?
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Copper at the cathode and oxygen at the anode.
Check 10
Why does the blue colour of copper sulfate solution fade during electrolysis?
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Copper ions are removed from the solution as copper forms.
Check 11
What is the test for hydrogen gas?
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A lit splint makes a squeaky pop.
Check 12
What is the test for oxygen gas?
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It relights a glowing splint.
Check 13
What is the test for chlorine gas?
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It bleaches damp blue litmus paper, turning it red first.
Check 14
What happens to a copper anode in copper sulfate solution?
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It dissolves, so it loses mass.
Check 15
What happens to a copper cathode in copper sulfate solution?
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Copper is deposited on it, so it gains mass.
Check 16
How is copper purified by electrolysis?
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Impure copper is the anode and pure copper builds up on the cathode.
Check 17
How do you electroplate an object?
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Make it the cathode, use the plating metal as the anode and an electrolyte of its ions.
Check 18
What is the half equation for hydrogen forming at the cathode?
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2H⁺ + 2e⁻ → H₂.
Check 19
What is the half equation for oxygen forming from hydroxide ions?
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4OH⁻ → O₂ + 2H₂O + 4e⁻.
Check 20
What is the half equation for a copper anode dissolving?
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Cu → Cu²⁺ + 2e⁻.
Check 21
Why is chlorine electrolysis done in a fume cupboard in school?
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Chlorine is a toxic gas.
Check 22
Which variables should you control when comparing solutions in electrolysis?
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Volume and concentration of solution, electrodes, voltage and time.
Check 23
What are three uses of the products of brine electrolysis?
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Hydrogen as a fuel, chlorine to disinfect water, sodium hydroxide to make soap.
Check 24
What colour is bromine in solution?
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Orange-brown.
Check 25
Why is electrolysis done with direct current?
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So each electrode keeps the same charge and the products stay separate.
Keep going with Electrolysis of aqueous solutions
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