Another interesting by one of my Secondary 4 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 hydrogen and oxygen is 2:1.
From the 4 multiple-choice options given, you can work out the answer to be A.
No. of moles of hydrogen = Mass / Mr of H2 = 1 g / (1 + 1) = 0.5 mol
No. of moles of oxygen = Mass / Mr of O2 = 8 g / (16 + 16) = 0.25 mol
Molar ratio of Hydrogen : Oxygen will be 0.5 mol : 0.25 mol which works out to be 2:1.
PS: Hope you will the above discussion useful to you. Do share it with your friends.
PPS: You can check out older posts on discussion of Electrolysis or Mole Calculations. Simple type in “Electrolysis” or “Mole Calculations” at the “type here to search” field located on the top right hand corner of this blog.
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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 ?