The mass of one mole of carbon-12 atoms is exactly 12 grams. This is because the atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom. Since one mole of any substance contains Avogadro's number of particles (approximately 6.022 x 10^23), one mole of carbon-12 atoms will have a mass equal to the atomic mass of carbon-12, which is 12 atomic mass units (amu), or 12 grams.