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PV=nRT
At constant number of moles of oxygen gas, we have:
P1V1T2=P2V2T1
Inserting values:
(1.2 atm)(257 mL)T2=(1 atm)(V2)(273K)
T2=222.5 K
Thus, the volume of oxygen gas at STP will be:
V2=(P1V1T2)/(P2T1)
=(1.2atm)(257mL)(273K)/(1atm)(222.5K)
=307.5mL