1. The problem statement, all variables and given/known data Consider the following gas phase reaction: 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= ? 2. Relevant equations n=PV/RT; M=mRT/PV; 3. 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?