Calculate the mass units of. 25 mol of carbon - 12 atoms.

The molar mass of the Carbon-12 from the periodic table is given to be 12.011 g. Multiply this mass with the number of moles given.

(0.25 moles) x (12.011 g / mole) = 3 g

Thus, the mass of 0.25 moles of carbon-12 is 3 g.