O Level Chemistry: Mole Concepts / Mole Calculations

23 Feb

Posted by: sean in: Chemistry Notes & Tips

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Mole Concepts is considered by many students to be one of the toughest topic in GCE O Level Examinations as well as GCSE Examinations (according to a close friend which is coaching under GCSE). In fact, students tends to give up the whole chapters when they go for their preliminary as well as GCE O Level papers. Actually, Mole Concepts and Chemical Calculations are manageable topics and in fact we can score in it, if we know the strategy to do so. Concepts is important in this topic whereby memorization is at a bare minimal. Yes! No need to memorise much! Do subscribe to this feed if you want to learn more about Mole Concepts and how to score confidently for this topic.

Let’s start with the in-depth discussion of the Mole Concepts Topic and see how we should approach some past years tough examination questions from top schools. We shall start with a manageable quesion that was once posted by a student:

Q) What is the volume at r.tp. of 10g of ethane (C2H6) gas?

Thought Process:

Find Relative Molecular Mass (Mr) of C2H6

Find the no. of mole present for 10g of C2H6

Use Molar Volume to find the volume at r.t.p

Suggested Answer:

Do participate and show us your working and answer in the “comment” section right below.

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9 Responses

Gary
23|Feb|2008 1

This should be easy.

Mr of C2H6=2(12)+6(1)
=30

No.of moles=mass/mr
=10/30
=0.333mol

Molar vol=No. of mol x 24dm3
=0.3333mol x 24dm3
=8dm3

I’ve a question…What is the difference between atoms,molecules and particles?

Thanks in advance!
Gary

Reply
sean
23|Feb|2008 2

Well Done Gary. Full Marks.

Like your way of presenting the answer. Clear and Concise.
Keep up the good work.

I will do a post in this blogsite within this 2 days on your question.
Stay tune and drop me a comment if you understand them.

Cheers
Sean

Reply
RIJA
24|Feb|2008 3

first i did @ my own and then i saw yours.so first of all we find no of moles.
no of moles=mass/mr

mr is 30.then 10/30.that makes it 0.333333.

then
vol of gas =no of molesxmolar gas vol.
=0.333333 x 24
=8

Sean : ” Your answer is correct but just ensure that you include your units with your answer. “

Reply
sean Reply:
April 24th, 2009 at 3:55 pm
Sean : ” Your answer is correct but just ensure that you include your units with your answer. “’

Regards,
Sean Chua
Master Trainer & Author
WINNERS Education Group

“Experience Learning with A Difference”

Reply
Varda
17|Apr|2009 4

i had a question actually.i did this question and i got the correct answer:)but the question i have is about the concerntration of a substance.another unit of concentration besides mol/dm3 is g/dm3.how is it possible?what do we do if we get a question like this.please i need help……tommorrow is my exam!!:(quick!!!!

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Varda
17|Apr|2009 5

i solved this question and i got the correct answer:)but i have another question besides this for the concentration of a substance.the unit of concentration is mol/dm3 and g/dm3.how is g/dm3 possibe.no of moles cannot be written as g.if we get question like thi……what do we do.do we convert the mass in grams in the no of molesand then solve the question?please i need help quick!!!!!!!!!!!

Reply
sean Reply:
April 24th, 2009 at 4:03 pm
Hi Varda,

Thank you for your comment.

There is indeed a way to change concentrations from mol/dm3 –> g/dm3

What you need to do is to use the MR involved. In this case, it will be MR of C2H6.

Next, apply the following formula: Conc. (g/dm3) = Conc. (mol/dm3) x MR

Try it out using the questions given to you by your school teachers.

Let me know how it works for you!

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

“Experience Learning with A Difference”

Reply
rija
18|Apr|2009 6

varda that’s strange……you wont get a question like that.the correct unit is mol/dm3.

Reply
sean Reply:
April 24th, 2009 at 4:03 pm
Hi rija,

check out my comments above to Varda.

Thanks.

Try it out using the questions given to you by your school teachers.

Let me know how it works for you!

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

“Experience Learning with A Difference”

Reply