 Thursday, September 20, 2007

O level Chemistry Question - Mole Concept

Halo all students preparing for your O Level exams,

Do try out the following Chemistry Question from the Top 5 Sec Schools in Singapore during their Preliminary Examinations:

Q) The number of atoms in 25g of Butane is:

A. 3 X 10^23
B. 3.6 x 10^24
C. 4.2 x 10^23
D. 4.2 x 10^24

All the Best
Sean
=)

Anonymous said...

Err, how do you even do this question in the first place? =/

lijun(: said...

MR(C4H10)= 4(12) +8 =58
no. of moles of C4H10=25/58
no. of molecules=(25/58)x(6x10^23)
no. of atoms=(25/58)(6x10^23)(14)=3.621x10^24

Sean Chua said...

Well Done Lijun. This is a Cedar Girl O level Prelim MCQ and understand that couple of classes were tricked.

For Anonymous, hope you understand what Lijun has done to get the answer. The tricky part is to multiple by 14. Avogadro's number is formally defined as the number of PARTICLES in 1 mole of substance. Do note that PARTICLES can refer to molecules, ions or atoms.

No. of molecules=(25/58)mole x(6x10^23)molecules/mole

No. of atoms=(25/58)(6x10^23)molecules x number of atoms in C4H10(14atoms)=3.621x10^24 atoms

Hope this helps. Let me know if you need more help in this.

Rgds,
Sean