Question

what will be the mass of 12 atoms of carbon

Answer 1

Atomic mass, the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, 1.992646547 × 10−23 gram, which is assigned an atomic mass of 12 units. In this scale 1 atomic mass unit (amu) corresponds to 1.660539040 × 10−24 gram.

Answer 2

Ok, so 1 atom of carbon weighs
1.660539040 × 10−24 gram.

But you need the mass of 12 atoms

So it will be ,

12 × 1.660539040 × 10−24 = 19.92646848gms

( I used the calculator :p)