The weight of one C -12atom is

We know that 12g of carbon (C-12) present in one mole of C-12.

One mol of C-12 has $6.023\times {10}^{23}$ number of atoms

Then, the weight of 1 atom of C-12 is $\frac{12\mathrm{g}}{6.023\times {10}^{23} \mathrm{atoms} \mathrm{of} \mathrm{C}-12}\times 1 atomof C-12 =1.9923\times {10}^{-23}g$The weight of one C-12 atom is $1.9926\times {10}^{-23}\mathrm{g}$