Answer :
The pressure unit is in atmospheres or atm. To confirm this from the ideal gas equation:
R = VP/nT
plugging in the standard values of pressure, temperature and molar volume,
R = (22.4L/mo)l(1atm)/(273.15K) = 0.0821
R = VP/nT
plugging in the standard values of pressure, temperature and molar volume,
R = (22.4L/mo)l(1atm)/(273.15K) = 0.0821