topscore question help please

[savvysearch]

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what is final pressure of gas that expands from 2L at 10C to 20L at 100C, it initially at 6 atm? Answer: 0.79 atm

I use V2=V1(P1/P2)(T2/T1); 20 = 2(6/x)(373/283)

Is there any other tricky way to do this faster?



When 14.250 moles PCl5 gas is placed in a 3 liter container and comes to equilibrium at constant temp, 40% of PCl5 decomposes : pCl5 <-->Pcl3 + cl2

what is Kc?

answer is (1.895)(1.896)/2.854

why?

also need help on below. This has always confused me:

The structure below has only two singlets:

CH3O---conjugated hexane ring----OCH3

why?

 

[Balki]

The first question you did corectly. That is the equation to use.
(P1)(V1) /(T1) = (P2)(V2)/(T2) Perhaps a little faster way to do this would be simply analizing what happens (i.e. if you double the temp then you double the volume) But you most likely won't have to do this on the real test, but will have to choose it from the answers that are presented in a "no calculation" way.
Second question.
So 40 percent of PCL5 decomposed and that is 5.7 moles (60 percent remained 8.55 moles)

So looking at the equation you see that the ratio of the decomposition materials is one to one so each of this products must be 5.7 moles.(Each of the products is presented in one mole) And it looks like this


8.55 5.7 5.7 (in moles)
PCl5 ---> PCl3 + Cl2

And it is in equilibrium so we are good. The only thing to do is to convert moles into concentration , because that's how K is computed..
[Pcl5] = 8.55/3 litres = 2.8
[PCl3] = 5.7/3 litres= 1.9
[Cl5]== 5.7/3 litres== 1.9

Kc = 1.9*1.9/2.8

Third question.
One singlet is coming from both methooxy groups, they are magnetically equivalent and don't split. The second is coming from the 4 hydrogens on the ring they are all equivalent as well.
If not convinced try using this cool interferiance on the web, you can actually draw your molecule and click analyse
http://www.upstream.ch/products/topnmr_demo.html
And you'll see that they indeed are identiacal.

Let me know if I can help you more. Good luck

 

[savvysearch]

Thank you so much~! You are a life saver.

 

[Balki]

Glad to help. 😀