If you know the volume of HNO3 that completely reacts with Na2CO3, then you know that both react completely and there really is "no" limiting reactant. That being so, you can take either the volume of HNO3 or Na2CO3, and given there molarities:
1) calculate the moles of either HNO3 or Na2CO3
2) use a molar ratio between either HNO3 or Na2CO3 and CO2
3) multiply by the molar mass of CO2 to get to grams of CO2
Note: Make sure your units are correct, is molarity mol/mL or mol/L? Are your volumes in mL or L? Make sure the units match up.