O Level Chemistry Question: Mole Concepts / Mole Calculations

15 Oct

Posted by: sean in: Chemistry Notes & Tips

A Chemistry Question that was posed by a local singapore student that is taking her O level this year.

Question:
Which of the following consists of the greatest number of atoms?
A. 4g of hydrogen gas, H2
B. 48g of solid magnesium, Mg
C. 71g of chlorine has, Cl2
D. 24 dm3 of argon gas, Ar at r.t.p.

Any takers??

PS: Leave your suggested answers in the “Comments” section below:

Rgds
Sean
SimpleChemComcepts
Learning Experience with A Difference

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9 Responses

illuminate-x
16|Oct|2007 1

I’m thinking it’s A…

No. of moles of H2 = 4/2 = 2 mol
No. of moles of Mg = 48/24 = 2 mol
No. of moles of Cl2 = 71/71 = 1 mol
No. of moles of Ar = 24/24 = 1 mol

We can rule out C and D, which leaves A and B – H2 obviously has more atoms than Mg, which leaves us with A ;D not very sure about this…

Reply
Cheney
16|Oct|2007 2

hmm. i think it’s A too.

not really sure, but since the avogadro constant states that ‘1 mol = amount of substance which contains 6.02 x 10^23 particles, where particles may refer to atoms, molecules, charges, ions or electrons’, then 2 mol of hydrogen gas probably contains 6.02 x 10^23 x 2 x 2 atoms, which magnesium only contains 6.02 x 10^23 x 2. This is because hydrogen is diatomic. or something like that. roughly la : )

Reply
Spirit-Breaker
18|Oct|2007 3

A or B.
I worked out for A and B and I got 2 moles.
For C and D it is out because I got 1 mole.
I’m not too sure…

Reply
Sean Chua
20|Oct|2007 4

Well Done!

Cheney answers is closest for his explaination.

This is indeed have to do with Avogadro’s constant.

Trick for this question is to pick up the fact that they are asking for ATOMS. H2 gas molecule has two atoms.

Sean
SimpleChemConcepts
“Learning Experience with A Difference”

Reply
Sherman Bok
12|Aug|2008 5

Just a question, if they are asking for atoms, shouldn’t we rule out the possibility of A and C? This is due to the fact that they exist as molecules. Although you can say we can calculate the “number of atom” from molecules, however due to the formation of covalent bonds, they are no longer atoms but molcules. Similarly, neither can you call an ion an atom. Hence, is there even the slightest possibility that the answer is B, with 2 moles of Magnesium (that is, 1.204 x 10^24 atoms), more than the 6.02 x 10^23 atoms of Argon? Thanks a lot, and please don’t flames me, thats just my take pertaining to the question.

Reply
sean
18|Aug|2008 6

Hi Sherman,

That is a good question from you. You are not wrong to signal out that molecules and ions are indeed different from atoms.

However, based on GCE O Level examination context, molecules and ionic compounds are essentially made up of atoms. As such, the questions is asking how many atoms made up a certain amount of molecules or ionic compounds.

As such, the answer still stands.

Hope this helps.

PS: By the way, which country are you from and which educational system/code do you follow?

Cheers
Sean Chua
Founder & Master Trainer of SimpleChemConcepts.com

Reply
Sherman Bok
18|Aug|2008 7

Singapore. Taking ‘A’ levels now but I think if the same question comes out the answer still stands. I sometimes just mull too much over certain questions.

Reply
arush
03|Jun|2009 8

the answer for all is the same
the no. of atoms in each case is equal

Reply
arush Reply:
June 3rd, 2009 at 10:13 pm
A is the answer

Reply