Here's why:

* Atomic Mass: The atomic mass of carbon is 12.011 atomic mass units (amu). This means one carbon atom weighs 12.011 amu.

* Mole: A mole is a unit of measurement that represents a specific number of particles (atoms, molecules, etc.). One mole of any substance contains 6.022 x 10^23 particles (Avogadro's number).

* Grams: The atomic mass of an element in amu is numerically equal to the mass of one mole of that element in grams. So, one mole of carbon weighs 12.011 grams.

Therefore, since one mole of carbon contains 6.022 x 10^23 atoms and weighs 12.011 grams, it takes that many carbon atoms to equal 12.011 grams.