AP Chem Stoichiometry involving gases help!?

Question

I need help with this one problem:

A gaseous mixture at 25 degrees C contained 1 mole of CH4 and 2 moles of O2 and the pressure was measured at 2 atm. The gases then underwent the reaction shown:

CH4(g) + 2O2(g) --> CO2(g) + 2H2O(g)

What was the pressure in the container after the reaction had gone to completion and the temperature was allowed to return to 25 degrees C?

The choices for the answer are

a. 1 atm

b. 2 atm

c. 3 atm

d. 4 atm

e. 5 atm

Please help!! thanks!!

Answer 1

Total pressure would still be 2 atm because you have the same amount of moles of gas on both sides of the equation and assuming the container is closed the volume won't be changing and the temperature is still the same so therefore the total pressure is still the same.

PV=nRT

Answer 2

b) 2 atm

No calculations required: 3 moles of gas to start and 3 moles of gas after reaction. No change in mols and no change in temperature. Therefore no change in pressure.

Answer 3

haha! I need stoicheometry help as well! if someone happens to answer my question, i'll re-edit this post and explain how I learned it- so you will know as well! :-)  good luck

http://answers.yahoo.com/question/index;...

Answer 4

The pressure was measured to be 2 atm, I presume, not "at" two atmospheres.  

PV=nRT or holding V & T constant kP = n

n to begin is 3

n at end is 3  oooppps red alert!red alert!phase change!phase change!

n is 1 plus the vapor pressure of the liquid water

ooops again! the equation reads that the water is gaseous...at 25C ? !...hmm...hmmmhmmm. sure.

Time to ridicule teacher...

Anyway for ideal gasses since n1 = n2 we can conclude P1=P2

--

a quick search reveals that vapor pressure of water at 25C is 0.03 atm

so the pressure would actually be closer to about 0.4 atm = 1/3 + 0.03

Answer 5

The pressure in the container (2 atm) should remain unchanged since the number of gas moles of product (3) equals the number of gas moles of reactant (3). No change in gas moles means no change in volume, and with no change in T, P can't change either.

Answer 6

Sneaky!

My first attempt: PV = nRT, but the number of moles of gas don't change, so 2 atm as at the start.

My second attempt: yes, but at 25° C water won't be a gas.  So you put in 3 moles of gas and got out only 1. So the answer should be 2/3

My third attempt: my second attempt was right.  I can only assume that whoever set this question didn't think it through properly (these things do happen).  Explain all this to your instructor.

Good luck.