Gas mixture composition of 78.0% nitrogen, 17.0% oxygen, % argon,% carbon dioxide. find the mass of nitrogen?

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

i believe it is c for plato