Comments on: O Level Chemistry – Mini Series on Tough MCQs Part 4

By: zainub

zainub — Fri, 24 Apr 2009 12:40:38 +0000

im from Pakistan….and im doin GCE o-levels….yeah thanx alot its all clear now….Infact i gave dat explanation dat u have given here to my skool teacher n she agreed to it….Thanx to u dat now all my classmates know da right answer…

By: sean

sean — Fri, 24 Apr 2009 07:19:36 +0000

Hi zainub,

Where are you from? Singapore Secondary School?

In GCE O Levels Syllabus, we will calculate the MR of the gases and compare to check their density (lightness).

You are right that main gases in AIR is nitrogen (N2) and oxygen (O2), with a percentage composition of about 78% and 21% respectively.

MR of N2 = 28
MR of O2 = 32

Average MR of N2 + O2 in AIR = (78/100 * 28) + (21/100 * 32)
= 21.84 + 6.72
= 28.56

If you compare it to the MR of Carbon Dioxide (CO2) of 44, you realised that AIR is “lighter” than carbon dioxide.

Hope the above explanation is clear to you.

Let me know how you think.

To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group

“Experience Learning with A Difference”

By: zainub

zainub — Tue, 07 Apr 2009 16:47:53 +0000

I am a bit confused. My chemistry teacher has taught us that Carbon dioxide is lighter
than air. Air has Mr greater than 60. The main gases in air are Nitrogen and Oxygen
and if we calculate their respective Mr they sum up to (28 + 32 = 60) 60 where as
the Mr of Carbon dioxide is 44. 44 < 60 therefore, carbon dioxide is lighter than air…

By: Seng Chiy

Seng Chiy — Mon, 10 Nov 2008 06:42:40 +0000

Yes, I am taking my O Level Chemistry Exams this year, however, I’m taking the IGCSE syllabus, not the GCE Chemistry syllabus. It’s roughly the same with just a few minor differences.

Thanks for the comment and I would try my best to improve myself at Chemistry.

By: sean

sean — Mon, 10 Nov 2008 05:11:33 +0000

Hi everyone,

Perhaps it will be good for me to sum all these up in one comment.

For MCQ question, we should use the Elimination Process to find the correct answer, based on information (clues) given in question.

Clue 1: Neither burns nor supports the burning of a splint
- Oxygen is eliminated since it supports burning of splint
- Hydrogen is eliminated since it burns in oxygen to form water vapour

Clue 2: Colourless Gas
- Chlorine gas is eliminated since it is a yellow greenish gas

Clue 3: Denser than Air
- Do note that Air consists 78% of Nitrogen and 21% of Oxygen
- As such we can take the relative molecular mass of Air as the Mr of Nitrogen (which is 28) since the majority of Air is made up of Nitrogen
- For Helium, it has a relative atomic mass of 4, and thus has to be eliminated.

Answer: We are left with Carbon Dioxide which should be the answer.

Double-Check: Carbon Dioxide has Mr of 44, which is higher than Air (thus denser than Air).

This is the sequence that i always ask my students to used.

Hope it works for you too.

All the Best To Your Academic Success in Chemistry,
Sean
Master Trainer & Author
http://www.SimpleChemConcepts.com