• Skip to primary navigation
  • Skip to content
  • Skip to primary sidebar
SimpleChemConcepts

SimpleChemConcepts

O-Level Chemistry Made Easy

Show Search
Hide Search
  • About Me
  • Tuition
    • Testimonials
  • Books
    • O-Level Chemistry TYS Book
    • O-Level Pure Chemistry Pocket Summary
    • Up Your Chemistry Grades Now! ebook
    • Pass With Distinction: Chemistry
    • Pass With Distinction: Science Chemistry
  • Videos
  • Download
  • Contact
You are here: Home / Chemistry Notes & Tips / O-Level Chemistry: Question on Electrolysis & Mole Calculations

O-Level Chemistry: Question on Electrolysis & Mole Calculations

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.

Related Articles:

  • O Level Chemistry: Challenging Mole Calculation / Electrolysis Questions
  • O Level Chemistry: Challenging Mole Calculation / Electrolysis Questions
  • O Level Chemistry – Mole Concepts / Mole Calculations
  • O-Level Chemistry: Question on Salts Solubility & Mole Calculations
  • O-Level Chemistry: Selective Discharge of Ions in Electrolytic Cells | Part 1: Position of Reactivity Series & Concentration Effect

Sean Chua
Sean Chua
Winners Education Centre Pte. Ltd.
Many know me as the Ten Year Series book author for JC A-Level H2 Chemistry and O-Level Pure Chemistry. My 19+ years of coaching experience (since 1999) with more than 1500 students from 180+ JCs and Secondary Schools has allowed me to understand the true reasons why students are not able to perform well in Chemistry. Most importantly, my strength lies in using everyday analogies (even grandmothers can understand!) to simplify abstract concepts. Also, my teaching methodology has been designed to be fun, enjoyable and effective for my students. If you want to have an enjoyable time learning Chemistry and being motivated to excel in Chemistry, contact me today at 98287357

Reader Interactions

Comments

  1. Daisy says

    January 21, 2019 at 3:10 am

    Hi
    I need to see more form 4 questions on electrolysis…
    Please

  2. SFS says

    March 21, 2016 at 1:15 am

    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.

  3. sean says

    August 13, 2013 at 3:38 pm

    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

  4. John Ng says

    May 9, 2013 at 12:31 am

    does this kind of questions only appear for pure chemistry ?

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Primary Sidebar

Join My Classes Today!

O-Level Chemistry Tuition

Buy My Books at Popular Bookstores!

Read my H2 Chemistry Blog here

A-Level H2 Chemistry Blog

Copyright © 2019 · SimpleChemConcepts.com · Talk to us at 98287357