How much CO2 gas is contained in a 12 gram CO2 cartridge?

You would need to know the pressure of the gas to answer that. Does the container specify the pressure of its contents? The gas law requires that you know the volume and pressure of the gas. 12 grams specifies its weight. And without looking up the specific cartridge, I don’t know if that’s the gross weight or weight of the empty container. If you can find both weights, you can also subtract and find the actual weight of gas, which would get you to its volume at a given pressure, say 1 atmosphere.

It does not say.

If we assume the 12 grams is the weight of the CO2, the magic constant you seek is 22.4 liters per one mole of gas at STP. Since CO2 is 12 + 32= 48 gm for one mole 22.4 liters that means one cartridge contains 12/48 *22.4 = 5.6 liters at STP. (standard Temp and pressure)

You can figure out the pressure by dviding the 5.6 liters by the volume of the cartridge. that will tell you the pressure in Bars (atmospheres). Multiply by 14.7 to get PSI. or 100 to get kPa.

Oops! I see that I said 12+32 =48 Dope! .It is 44 obviously
So 12/44 * 22.4 = 6.1 liters
Sorry. It is late.

CO2 has a density of 1.977 kg/m³, or 1.977 g/l, ergo 12g of CO2 at normal pressure means 6.07l of CO2.

@worriedguy and @ragingloli got 6.1 liters and it would fill a different size balloon depending on the temperature and the pressure holding it in the balloon. So you can’t really get an answer without an actual experiment because you also get to decide how flacid or expanded you want balloon skin. The balloon skin may influence the result too. So you almost need an answer ( or at least more info) before you work the result. Interesting !