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Blazing goIITian

 Joined: 9 May 2009 Post: 766
8 Mar 2010 20:59:05 IST
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Chem. exam doubt
Engineering Entrance , JEE Main , JEE Main & Advanced

Today i had confusion in one question in the chemistry board paper. Q19 ( Set 1).

Ques. was like this -

Determine the crystal structure(SCC, FCC or BCC)-

radius of atom was given, density was given and Molar mass was given. avogadro's No. we know.

We can use the formula, d = M X Z / No X a3

The problem was no value of edge length and we can do find it from radius but for that we need to know which crystal lattice is it - SCC, FCC or BCC. But dat is wat is asked in the question.

Could someone clarify dis doubt.

=======>>>>  I solved if by assuming that it is FCC and calculated edge length and then verified the assumption by finding value of Z.

Blazing goIITian

Joined: 21 Aug 2008
Posts: 380
11 Mar 2010 22:00:47 IST
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i too had the same set...bt couldn't do it.. jst bcz edge length was not given....

Hot goIITian

Joined: 16 Oct 2009
Posts: 120
11 Mar 2010 22:39:06 IST
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I too got same set and having lot of confusion.....Also comparing with question of solid state with other sets..I think we should get 3 marks...If we also prove it for SC,BCC,FCC individually....then also calculating cube of 177 is impossible !!!!!!!!!

Forum Expert
Joined: 19 Feb 2009
Posts: 1958
12 Mar 2010 00:30:02 IST
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Relation between radius and edge length of FCC,BCC,SCC......

For BCC : 4r = root(3).a

For FCC : 4r = root(2)a

For SCC ; r = a/2

Now Apply the formulae of density put all the values.....and get a relation as such...

Z = K.a^3 --------(1)  K is by solving all the given values...

Now r is given....

Then separately on the page calculate  a for BCC,FCC,and SCC...

BCC             a=?

SCC             a=?

FCC              a=?    { Calculate a by using the a and r relation}

And one by one put in (1)

If u have used the value of a from BCC then Z must come equal to 2....and if it is not coming then Solid will not be BCC,

Again use a from SCC  and calculate Z,,if Z comes equal to 1 then it is SCC if not then try for other...

So hopefully it will give the correct answer,,,

Assuming first as FCC is not the correct way,,,,,,YOu have to show that if FCC is the answer then separately BCC and SCC couldn't be the answer....

I am wondering !!!!!!! How much mark this question carrying? Please any body....

Blazing goIITian

Joined: 9 May 2009
Posts: 766
12 Mar 2010 11:39:31 IST
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I did the same way u said yagyadutt sir. But this ques. was of just 3 marks. I think they by mistake mentioned radius rather than edge length.

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