IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: Saladin on May 10, 2010, 08:54:39 am

Title: Moles Problem!
Post by: Saladin on May 10, 2010, 08:54:39 am
Hey, I really dont get this. The question is here (http://www.mediafire.com/?uzjzw1yzzit).

This is how I did it:

(c)(i)

(http://www.sitmo.com/gg/latex/latex2png.2.php?z=100&eq=%5Cfrac%7B20%7D%7B126.9*2%7D%3D0.079%20)

(ii)

As the ratio of I2 and CH3CH2I is 1:1.

Therefore, it should be something like this:

(http://www.sitmo.com/gg/latex/latex2png.2.php?z=100&eq=0.079*156%3D12.3)

But this is not the answer. I really not understand how this all works!
Title: Re: Moles Problem!
Post by: nid404 on May 10, 2010, 09:23:50 am
2P + 3I2 ? 2PI3
3CH3CH2OH + PI3 ? 3CH3CH2I + H3PO3

Look carefully at the ratio

1:1?

1:2 :) 

Title: Re: Moles Problem!
Post by: Saladin on May 10, 2010, 09:27:49 am
Quote from: nid404 on May 10, 2010, 09:23:50 am
2P + 3I2 ? 2PI3
3CH3CH2OH + PI3 ? 3CH3CH2I + H3PO3

Look carefully at the ratio

1:1?

1:2 :) 



Thats what I dont get, how is it 1:2?
Title: Re: Moles Problem!
Post by: Meticulous on May 10, 2010, 09:28:04 am
Exactly..look carefully at the ratios

2PI3 ----> I2

PI3 ---> 3CH3CH2I

first find the moles of PI3 : 0.0788* 2/3 =0.05253

now find iodoethane mole: 3* 0.05253= 0.1576

therefore, mass= 0.1576* 156= 24.6 g
Title: Re: Moles Problem!
Post by: princess12 on May 10, 2010, 09:29:41 am
that is the thing i donot get it also how the ratio come always
Title: Re: Moles Problem!
Post by: tmisterr on May 10, 2010, 09:31:12 am
look at the first equation, 3 moles of Iodine form 2 moles of PI3 so 0.079 will form (0.079*2/3) moles of PI3. And now the second equation, one mole of PI3 will form 3 moles of Iodoethane so ((0.079*2/3)*3)=0.158 moles of iodoethane which is equal to 24.648 grams
Title: Re: Moles Problem!
Post by: nid404 on May 10, 2010, 09:33:56 am
Basically

3moles of I2 gives 2 moles of PI3

the next reaction uses only one mole of PI3   ---> multiplying this eqn by 2 will give you the ratio


 
Hope ur gettin me  :-\