Another interesting question by one of my Secondary 4 O-Level Pure Chemistry students. This time it is on Electrolysis & Mole Calculations.
Let’s take a look at the question.
Question:
If dilute H2SO4 is electrolysed using graphite electrodes, the products obtained will be in the proportions of:
A) 1 g of hydrogen : 8 g of oxygen
B) 1 g of hydrogen : 16 g of oxygen
C) 2 g of hydrogen : 8 g of oxygen
D) 1 dm3 of hydrogen : 2 dm3 of oxygen
Now, go take out a piece of paper and attempt the question before you scroll down to look at the suggested solution.
Suggested Solution:
Step 1: Ions available in electrolyte:
H+(aq), OH–(aq) and SO42-(aq)
Step 2: Oxidation occurs at the +ve Anode
OH– (aq) ions is selectively discharged based on the following half equation.
4OH–(aq) –> 2H2O(l) + O2(g) + 4e–
Step 3: Reduction occurs at the -ve Cathode
H+(aq) ions is selectively discharged based on the following half equation.
2H+(aq) + 2e– –> H2(g)
Step 4: Overall equation for the electrolytic cell
2H2O(l) –> 2H2(g) + O2(g)
As you can see the molar ratio between H2 and O2 is 2:1.
Assuming that 1 mol of O2 is being produced. 2 mol of H2 will be produced.
Mass of O2 produced = Mole x Molar Mass = 1 x (16 + 16) = 32 g
Mass of H2 produced = Mole x Molar Mass = 2 x (1 + 1) = 2 g
Mass ratio of H2 : O2 will be 2 g : 32 g which can be simplified to become 1 g : 16 g.
As such, answer is Option B.
I hope you find the content easy for your understanding and that this article will be part of your Chemistry Notes to master Electrolysis.
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Hi
I need to see more form 4 questions on electrolysis…
Please
I really found this question useful but overall the problem I face in chemistry is that i can not gain good marks in it. Like the mole calculations and few other valency type of things kill me.
Hi John,
I reckon you should be based in Singapore.
Yes, this type of question should only be seen in Pure Chemistry examination because “Electrolysis” topic is not covered in the Combine Science Chemistry syllabus.
Hope you find this website useful.
Regards,
Sean Chua
does this kind of questions only appear for pure chemistry ?