A few students from my weekly chemistry sec 4 classes have been asking me about the following Multiple Choice Questions (MCQ) on Electrolysis, more specifically on Electrolytic Cell.
It seemed like many secondary schools in Singapore had set this question in the recent school’s O Levels Preliminary Exams.
Question:
In an? electrolysis experiment, the same quantity of electricity deposited 16g of copper and 6g of titanium.
What was the charge on the titanium ion?
A. 1+
B. 2+
C. 3+
D. 4+
What do you think?
Clue: Although this is question is on Electrolysis, you need to realise that it is calculation-based and you should be thinking of Mole Calculations at the same time.
Work it out on your own, before you click the link below to check the answer and my suggested solution (with thought process).
Answer : D
Thought Process:
1. Has to write out Reduction Half Equations for discharge of both Copper & Titanium Ions
2. With mass of metal given, you need to calculate the No. of Moles
3. Set up a mathematical relationships be the 2 half equations based on the fact that same quantity of electricity is used
Suggested Solution:
Mole of Copper = Mole / Ar of Cu = 16g / 64 = 0.25mol
Reduction Half Equation: Cu2+ + 2e– –> Cu
Based on molar ratio, 1 mol of Cu requires 2 mol of electrons
Mole of electrons = 0.25 mol x 2 = 0.50 mol
Mole of Titanium = Mole / Ar of Ti = 6g / 48 = 0.125 mol
Let the charge of Titanium Ion be m, therefore;
Ti m+ + me- –> Ti
Mole of electrons = 0.125 mol x m = 0.125m mol
Since question states that ‘same quantity of electricity are deposited’,
it also means that the number of moles of electrons must be the same.
Hence, 0.125m = 0.5
m = 4
Charge of Titanium Ion will be 4+ and answer is D.
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 2010 during the 1-week September School Holidays. Check it out! Seats are extremely limited.
I think it’s
Ce2+
WOW..you are really nice.. can you list more questions with solutions?
If an aqueous solution of cerium salt, 35g of cerium (atomic no of Ce=58) is deposited at the cathode by 1 mole of electrons. What is the formula of the cerium ion ? Please help
What remain is questions comes at different form some even ask to calculate d mass of element deposited on cathode
Is the answer A? Titanium as +1??? I tried using the method using the ‘elements box’.
Hi R. Was,
You are right that copper has two ions Copper(I) and Copper (II). It is a transition metal and they have variable oxidation states – thus the presence of different ions.
In this question, no clue was given whether it is Copper(I) and Copper (II) ions involved. We chose it to be Copper (II) ion but it is the predominant ions.
For GCE O Levels Syllabus, students were taught to take copper ion to be Cu2+ if nothing is mentioned in the question.
Just an add-on: How do we know that zinc chloride is ZnCl2? i.e. the cation formed by zinc is Zn2+? Cos Zn2+ is the predominant ion.
Feel free to add on any comments =)
Copper has at least 2 ions, copper(I) and copper(II); this would make a difference in the answer. Or do you expect the student to know all of the oxidation species real or hypothetical for titanium?