What is the difference between a container filled with carbon-12 and a container filled with carbon?

Elemental carbon has different isotopes. The most common isotopes are carbon twelve, thirteen and fourteen. Carbon 12 and 13 are more stable isotopes. 14 is radioactive with a half life of 5700 years. Different isotopes have slightly different atomic masses. A container filled with only 12 would have a different weight than one with naturally occurring carbon.