Best Total number of moles n = PV/RT = (12.5atm x 10L)/(0.082 x 298K) = 5.1154 mole
30g of oxygen = 30g/32g/mol = 0.94 mole. Also 30g of nitrogen = 30g/28g/mole = 1.07 moles.
Let the number of moles of CO2 be = c,
0.94 + 1.07 + c = 5.1154. Hence c = 3.105 mole = number of moles of CO2
Mass of CO2 = 3.105 moles x 44g/mole = 136.62g ~ 137g