An atmospheric chemist studying the pollutant SO2, places a mixture of SO2 and O2 in a 2.00 L container at 800k and 1.90 atm. when the reaction occurs, gaseous SO3 forms, and the pressure falls to 1.65 atm. How many moles of SO3 form? using PV=nRT why can't I directly find number of SO3 moles formed using Pressure 1.65 atm?