Ek chem lecture 2, ?-27

[justwantin]

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27. Ammonia burns in air to form nitrogen dioxide and water.

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 temperature.)

A. 4 atm
B. 6 atm
C. 11 atm
D. 12 atm
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I read the explaination, they say the number of gas moles in extra info. Further they say, if the container began at 11atm then each gas is contributing a pressure in accordance with its stoichio. coeffi. When the reaction runs to completion, the only gas in the container is NO2, so the partial pressure of NO2 is the total pressure. The volume of the container remains constant, so the pressure is in accordance with the stoi. coeff. of NO2.

My question is why are the moles given extra info? Why dont we assume 8 moles of NO2 are formed? and that the 8 moles formed contribute the partial pressure. why is it 4 and not 8?

Thanks in advance.

 

[supras2kracer]

If you look at the gas law PV=nRT you know T is constant so it depends on the number of moles. Volume is constant as well since it's a closed container. This means that P=n

They give you the number of moles to try and throw you off track. You have to just think about the equation. Think of it as it takes 4 moles of NH3 to react with 7 moles of O2. That will always produce 4 moles of CO2.

Using the number of moles they gave, you'd have to figure out the limiting reagent and find the theoretical yield. You get 8 moles like you said but you also have 12 moles of water. So its mole fraction in that case would be .4. The total pressure is just the number of moles made since P=n. So .4 X 10=4 atm

 

[jajo]

Ignore the number of moles. If the total initial pressure is 11 atm, you can calculate using mole fractions (coefficients) that the initial partial pressure of NH3 is 4 atm and the initial partial pressure of O2 is 7 atm. Given these initial partial pressures and looking at the equation, if the reaction were to go to completion, the total pressure of the products would be 4 atm (since this the total number of moles of gases in the products). This then is also the partial pressure of NO2 since it is the only gas.


Also with reference to the above explanation, when finding mole fractions for use in partial pressure calculations, I'm almost positive only moles of gases should to be considered; so water should be ignored.

Hope this helps. 👍