First, we convert the pressure in kPa to atm. That is,
(215 kPa) x (1 atm/ 101.325 kPa) = 2.122 atm
Then, determine the amount of carbon (C) in mol by assuming that it is an ideal gas.
n = (2.122 atm)(75 L) / (0.0821 L.atm/mol.K)(-25 + 273.15)
n = 7.81 mols
The reaction is expressed as,
4KO2 + 2CO2 --> 2K2CO3 + 3O2
From the equation,
(7.81 mols CO2) x (4 mols KO2/2 mols CO2) = 15.62 mols KO2