masses of atoms are the sum of neutrons and protons. Atomic mass given for the element is the weighted atomic masses of the isotopes depending on the abundance of the isotopes.
Atomic masses of the elements making up Na₂C₂O₄ are as follows;
Na - 22.98 a.m.u
2 atoms of Na - 22.98 x 2 = 45.96
C - 12.01 a.m.u
2 atoms of C - 12.01 x 2 = 24.02
O - 15.99 a.m.u
4 atoms of O - 15.99 x 4 = 63.96
Sum of the atomic masses = 45.96 + 24.02 + 63.96 = 133.94
Mass is 133.94 g/mol
