Answer : The number of moles of oxygen in the reaction vessel is, 0.761 mole.
Explanation :
To calculate the number of moles of oxygen gas we are using ideal gas equation as follows:

where,
P = Pressure of
gas = 84.8 atm
V = Volume of
gas = 0.217 L
n = number of moles
= ?
R = Gas constant =

T = Temperature of
gas =

Now put all the given values in above equation, we get:


Therefore, the number of moles of oxygen in the reaction vessel is, 0.761 mole.