Question

what is one atomic mass?

Answer 1

one atomic mass is equal to 1/12th the mass of C-12 atom.
1a.m.u(atomic mass unit)=1.6604*10^(-24) gms

Answer 2

Atomic mass unit is 1/12th the mass of a carbon -12 atom. 

1 a.m.u =1.66 x 10 raise to minus 24 gms