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 SO₄^2-(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 / M_r of H₂ = 1 g / (1 + 1) = 0.5 mol

No. of moles of oxygen = Mass / M_r 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
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• 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

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