Comments on: O Level Chemistry Question: Electrolysis / Mole Concepts / Mole Calculations

By: sean

sean — Wed, 05 May 2010 16:11:09 +0000

Hi Saud,

Check my answers above.

Cheers!

By: Saud

Saud — Sat, 27 Feb 2010 05:48:25 +0000

A
6dm cubic
finding moles of aluminium that make up to 0.333 as 9/27=0.333moles,
so,
as the charge on an aluminium ion is +3 so three moles of electricity i-e electrons is used to discharge it,
this makes the moles of electricity to be 3 X 0.333 = 0.999moles,,

now taking the oxygen ion, it has a charge of -2, i-e, one mole of an oxygen ion gives 2 mole of electrons/electricity……………

so, 0.999 moles of electrons get liberated from how much oxygen ions??…………

this makes it to be 0.999/2 = 0.4995 moles of oxygen ions………

one ion of oxygen is written as O(-2 ) and one mole of oxygen is written a O(2)……………………..so 0.4995 moles of oxygen will liberate 0.4995/2=0.24975moles of oxygen gas…………………..

corresponding to it, the volume of oxygen gas liberated at the anode will be as M.G.V / moles of oxygen,

i-e,

24 X 0.24975=5.994dm cubic which rounding off makes as 6dm cubic…………………………. hope im not wrong in my calculations

By: waleed bin qasim

waleed bin qasim — Fri, 09 Oct 2009 16:27:39 +0000

welldone sir u r gr8 teacher. Allah would give u suffice blessings

By: Sean Chua

Sean Chua — Sun, 30 Sep 2007 11:02:00 +0000

Hope you all understand the working to the answer for this question. Students tends to skip the steps on working out the half equations for the cathode and anode reactions of the electrolysis. Rmbr to use both half reactions and then get the overall reactions. It is only then you can use mole concept to solve the question. Difficulty? Not too bad. But you need to be exposed to such question in case it comes out in this year O Level.

Sean
SimpleChemConcepts

By: Sean Chua

Sean Chua — Sat, 29 Sep 2007 16:55:00 +0000

Hi Sirong and Platinum,

Both answers are wrong. This MCQ question requires us to think of a few more steps to get the answer. In the question, it involves Electrolysis, where Al metal is formed from Al3+ ions, and 02 gas is formed from the dilute sulphuric acid present.

First we need to look at the two half equations:

Al3+(aq) + 3e- => Al(s) ---- (1)

from dilute H2SO4, 02 gas will be liberated by the OH- ions present in water (dilute acid contains water), and equation is:
4OH-(aq)=>O2(g) + 2H20(l) + 4e- --(2)

Multiply Eq (1) by 4, and Eq (2) by 3, this will allow the electrons to be the same, and allows us to combine to form overall equation of:

4Al3+ + 12OH- + 12e- => 4Al + 3O2 + 6H20 + 12e-

Mole of Al liberated=9/27=1/3 mole
Based on overall equation, 4 mole of Al will gives 3 moles of O2.
Thus, 1/3 mole of Al will give 1/4mole of of O2.
Vol of O2 liberated = 0.25mole x 24dm3/mole = 6dm3

Ans: A

Hope this is clear.
Sean
SimpleChemConcepts