## Friday, September 28, 2007

### Chemistry Question - Electrolysis/Mole Concept

Hey student,

Here is another one for you to think about. Yup. O Level Prelimary Chemistry question. Think about it and let me know.

Q)A certain quantity of electricity liberates 9g of aluminiun. The volume of oxygen liberated at r.t.p from dilute sulphuric acid by the same quantity of electricity is:

A)6 dm^3
B)8 dm^3
C)12 dm^3
D)24 dm^3

Happy trying. Rmbr: Best thing is to write it down and play around with the equations. You will see the light in the tunnel....

Sean
SimpleChemConcepts

sirong said...

is the as D ? cos any gases at r.t.p is always 24 dm^3 ? am i right ?

platinumwinter said...

amt of aluminum liberated=9/27
amt of electrons=9/27*3=1 mol (because Al3+)
4OH- => 2H2O + 2O2 + 4e-
1 mol e- means 0.5mol O2 means 0.5*24=12dm3 O2 liberated so ans is
C, 12dm^3
isit correct? someone pls verify

robobooga said...

anyone can give the answer for this qn?

Sean Chua said...

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

Sean Chua said...

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

Anonymous said...

sorry, but why is it sulphuric acid? my first thought when reading the qn is that the electrolyte is aluminium hydroxide.
will appreciate it if you explain.