O Level Chemistry: Mole Concepts / Chemical Calculations

19 Mar

Posted by: sean in: Chemistry Notes & Tips

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This one may require abit more thinking to solve the question. Past years preliminary examination questions from local school.

Question:

Which of the following has the same number of atoms as 6 dm^3 of chlorine gas?

a) 20g of argon

b) 0.25g of hydrogen

c) 23g of sodium

d) 8.5g of ammonia

Show your working and reasoning on how you derive to the answer.

40 more days to Singapore Mid-Year Examinations…Stay focused!

“We all fail.
But it is not failing that hurts.
What hurts is knowing that you didn’t give your best.”
~ By Andrew Matthews “Happiness in a Nutshell”

Sean

PS: Check “Comments” below for Suggested Answers.

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6 Responses

Jen
20|Mar|2008 1

hello.

My answer would be either be A/B

no of moles of Cl2
= vol of gas/molar volume
= 6/24
=0.25 mol

no of atoms in Cl2
= no of moles x 6 x 10^23
= 0.25 x 2 x 6 x 10^23
=0.5 x 6 x 10^23

(i multiply by 2 because in 1 mol of Cl2 molecule, it contains 2 chlorine atoms)

no. of moles of Ar = 20/40= 0.5 mol
no. of atoms of Ar = 0.5 x 6 x 10^23

no of moles of H2= 0.25/1= 0.25mol
no of atoms in H2= 0.25 x 2 x 6 x 10^23
Hydrogen refers to H or H2?

no of moles in sodium= 23/23= 1 mol

no of moles in NH3= 8.5/17=0.5 mol
no of atoms in NH3= 0.5 x 4 x 6 x 10^23

i am not too sure for this question…

Reply
Gary
20|Mar|2008 2

Hello Jen,

Answer should be A .

Like what you said,Chlorine has 2 atoms because its Cl2.As such,this should apply to hydrogen too.Right Sean?

But remember,when you find the number of moles of H2…it should be 0.25/2 not 0.25/1.Otherwise,your working is the same as mine! =D

Reply
Jen
21|Mar|2008 3

Hi Gary,

Oh! no wonder I could not get the answer. ok, thank you for correcting me!

Reply
sean
21|Mar|2008 4

Hi Gary,

You are spot on for this one! The tricky part of this question is indeed to realise:
1) That “Hydrogen” means H2 gas molecule
2) Mr of “Hydrogen” is 2 instead of 1 (alot of students missed out on this part)

I comgratulate you on your participation. The greatest way of learning is through
active participation – where the knowledge will be in-grained into your cellular
structures.

Keep up the good work!

One who Conquers Oneself,
Sean

Reply
sean
21|Mar|2008 5

Hi Jennifer,

I congratulate you on your active participation and willingness to open in an open mind. The greatest way of learning is through active participation – where the knowledge will be in-grained into your cellular structures and be life-long.

I am glad that both Gary and yourself and actively correcting and teaching each other. Hope you are enjoying your preparation and find this platform useful.

Keep up the good work!

One who Conquers Oneself,
Sean

Reply
Student Blog Project :: Effective Study Tips, Strategies, and Methods » Blog Archive » Chemistry Question - Mole Concepts/Chemical Calculations
31|Mar|2008 6

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