Gen Chem -- EK

[Just Joshin]

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This is probably real simple and I'm makng it harder than it is. If someone can explain it to me like I'm 2, I'd appreciate it because I can't wrap my mind around the explanation in the book:

4NH3 (g) + 7O2 (g) ---> 4NO2 (g) + 6H2O (l)

If 8 moles of NH3 are reacted with 14 moles of O2 in a rigid container with an initial pressure of 11 atm, what is the partial pressure of NO2 in the container when the reaction runs to completion? (assume constant temp)

(A) 4 atm
(B) 6 atm
(C) 11 atm
(D) 12 atm

The answer is A, but I don't get it.

 

[bluemonkey]

This is probably real simple and I'm makng it harder than it is. If someone can explain it to me like I'm 2, I'd appreciate it because I can't wrap my mind around the explanation in the book:

4NH3 (g) + 7O2 (g) ---> 4NO2 (g) + 6H2O (l)

If 8 moles of NH3 are reacted with 14 moles of O2 in a rigid container with an initial pressure of 11 atm, what is the partial pressure of NO2 in the container when the reaction runs to completion? (assume constant temp)

(A) 4 atm
(B) 6 atm
(C) 11 atm
(D) 12 atm

The answer is A, but I don't get it.

This problem is testing Avagadro's Principle, where n1/v1=n2/v2. You are given the initial number of moles, the final number of moles, and the the initial pressure...

 

[Just Joshin]

I'm sorry to be dense, but what? I don't understand how you can do it with n1/v1=n2/v2

 

[BloodySurgeon]

Hmm... Yea the wording is really bad... It is saying that the inital pressure is 11atm After 22 mols of gas has been added.

8NH3 (g) + 14O2 (g) ---> 8NO2 (g) + 12H2O (l)

Partial pressure of NO2 == Ptotal because the rxn goes to completion and liquids don't exert a partial pressure.

Since PV=nRT and V,R,and T are constants... P/n = constant.

P1/n1 = P2/n2

11atm/22moles = P2/8moles

P2 = 4atm

 

[Just Joshin]

Thank you!!! Thank you!!! Thank you!!!

 

[bluemonkey]

I'm sorry to be dense, but what? I don't understand how you can do it with n1/v1=n2/v2

Yeah, sorry I read your post so quickly...I misread constant volume for pressure...

 

[BTC]

you can do it quicker. If you realize there are 22 mols of gas in a container at a pressure of 11 atm, how much pressure would 8 mols of gas be in the same container? its a 2:1 relationship.