O Level Chemistry: Challenging Mole Calculation / Electrolysis Questions

In the previous post, we have discussed on 1 MCQ question on Electrolysis that requires basic Mole Calculations strategy to solve.

Do note that Electrolysis topic is only applicable to students taking:

• GCE ”O” Level Pure Chemistry
• Integrated Programme (IP) – it will be classified under Electrochemistry chapter
• IB programme
• IGCSE and other equivalents

It is NOT applicable to GCE ”O” Level students taking Combine Science syllabus.

Today, we shall continue to look into another challenging Electrolysis question.

Question:

Which of the following requires the largest number of electrons for discharge?

A. 4 mol of Al³⁺ (aluminium) ions

B. 5 mole of OH^– (hydroxides) ions

C. 4 mol of O^2- (oxides) ions

D. 10 mol of Na⁺ (sodium) ions

Try it out! Once you have an answer, proceed to click the link below to see if you have gotten the correct answer. Suggested answer and thought process is given also.

Thought Process:

1. Half equations are required to see how many mole of electrons is required for discharge equation

2. Using Molar Ratio comparison, find out the actual mole of electrons required for “no. of mole of ions” given in each choice

Suggested Answer:

A) Al³⁺(aq) + 3e^–???Al(s)

1 mol of Al³⁺(aq) ions require 3 mol of electrons to discharge

Thus, 4 mol of Al³⁺(aq) ions require 3 x 4 = 12 mol of electrons to discharge

B) 4OH^–(aq) ??2H₂O(l) + O₂(g) + 4e^–

4 mol of OH^-(aq) ions require 4 mol of electrons to discharge

Thus, 5 mol of OH^–(aq) ions require 5 mol of electrons to discharge

C) 2O^2-(aq) ??O₂(g) + 4e^–

2 mol of O^2-(aq) ions require 4 mol of electrons to discharge

Thus, 4 mol of O^2-(aq) ions require 8 mol of electrons to discharge

D) Na⁺(aq) + e^–?? Na(s)

1 mol of Na⁺(aq) ions require 1 mol of electrons to discharge

Thus, 10 mol of Na⁺(aq) will require 10 mol electrons to discharge

With the above mole of electrons comparison, (A) has the largest number of moles of electrons required for discharged.

Common Error: Many students will choose (D) because they simply look at the “10 mol” and assume it has the largest no. of electrons directly.

So far so good for you? Let me know your thoughts on this question and the suggested solution. I would love to hear from you.

If you have a different way of solving this question, feel free to write down your thoughts on the ?Leave A Reply? section below.

PS: If you are finding it difficult to solve any mole calculations ? related questions, there is an upcoming Mole Calculations Mastery Workshop?during the June School Holidays. Check it out! Seats are extremely limited.

Related Articles:

• O Level Chemistry Question: Chemical Bondings
• O Level Chemistry – Definitions of Electrolysis
• O Levels Chemistry Tips Blog Marathon – Day 5
• O Level Chemistry: Mole Concepts / Chemical Calculations
• O Level Chemistry – Strategies to Predict Products of Electrolysis for Aqueuous Solutions