what is the mass of grams of 100 atoms of the carbon-12 isotope?

Assume 100 atoms has no uncertainty. Find the mass of 100 C-12 atoms.

The carbon-12 isotope is arbitrarily assigned a mass of exactly12 amu.
1 amu is 1.66053904x10^-24 g (1g divided by Avogadro's constant, 6.02214086 mol⁻¹. Avogadro's constant is expressed to nine significant digits.)

100 atoms C-12 x (12 amu C-12 / 1 atom C-12) x (1.660539040x10^-24g / 1 amu) = 1.99264848x10^-21 g ......... to nine significant digits

The mass is given to nine significant digits because that is the precision of Avogadro's constant.

1 mole of C-12 isotope contains 6.022 × 10²³ C-12 atoms.
Number of moles of C-12 isotope = 100/(6.022 × 10²³) = 1.661 × 10⁻²² mol

Molar mass of C-12 isotope = 12 g/mol
Mass of C-12 isotope = 12 × (1.661 × 10⁻²²) = 1.993 × 10⁻²¹ g