Find volume of a mixed gas at STP

[iconwin]

Homework Statement

Consider the following gas phase reaction:
http://scholar.uh.edu/webct/RelativeResourceManager/Template/Imported_Resources/20053H_CHEM1331_08946_QIZ_200511041405291131475017966%20folder/QIZ_6442871_M/my_files/test_images/f5g1q49g1.gif
400. mL of NO at STP is reacted with 500. mL of O2 at STP. Calculate the volume of the reaction mixture at STP after the reaction goes to completion.

V of NO= 400.mL= .400L
V of O2= 500.mL = .500L
At STP:
P= 1atm
T= 273K
V= 22.4L/mol of gas
V of mixed gas NO2= ?

Homework Equations

n=PV/RT;
M=mRT/PV;

The Attempt at a Solution

1, I added two volumes of gases together to get the total volume.
.400L+.500L= .900L

2. I calculated the mol of NO2 at STP
n= PV/RT= (1atm*22.4L)/(0.0821L*atm/K*mol)
n= 0.9994 mol of NO2

3. Took that mol multiply with .9L to get the Volume of the mixed gas NO2, I got ~900mL
But the right answer is 700mL. Is there any idea on what I did wrong?

 

[Borek]

The gases reacted. You end with two gases - but you need to calculate their amounts with simple stoichiometry.

 

[iconwin]

Thank you! I'll try again!