Does CO2 have mass

Of course! It might not seem like it, but all gases have mass (as with all solids and liquids...). If it has a chemical structure, it has mass.

If I weigh a balloon, then fill it with my own breath and reweigh it, it will now weigh more. Why? Because the CO2, N2 and O2 (among many other little bits) is the air we breath. It all has mass and it all has weight, so when we put it inside the balloon, the balloon becomes heavier, like putting Apples in a basket.

Note: This wouldn't be true for H2 or He2 since they are lighter than atmospheric air and so they 'float'...but nevermind that for now.

By looking at a Periodic Table, we can see how much each element weighs using the only number there that has decimal places. So, for example, Hydrogen mass is roughly 1 and carbon mass is roughly 12. There's all kinds of intricacies here, but learning the basics is more important.

So...CO2. We have 1 carbon and 2 oxygens. Carbon mass is roughly 12, Oxygen mass is roughly 16. (Two oxygens mass is roughly 32, then).

So 12+32 = 44. And the units for this is da (daltons) or amu (Atomic Mass unit).

That's about it. da and amu are just units of measurement, like a gram...but a lot smaller. Often we don't even put the unit and just say "mr".

Like I would write "mr of 44". Just means the molecule weighs 44 da or amu...but the units are used so universally that I don't bother writing them. That's just a note if you happen to see mr used or no units used. But I'd advise to use the units because using them is never wrong...omitting them can be.