O-Level Chemistry: Electrolysis & Mole Calculations Question

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 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 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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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