What is the Correct Volume of the Reaction Mixture at STP After Completion?

[iconwin]

Homework Statement

Consider the following gas phase reaction:
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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!