Molecular mass of NaCl = (1 x 23 + 1 x 35.5) u = 58.5 u
Atomic mass of sodium = 23 u
Mass per cent of Na = (Atomic mass of Na / Molecular mass of NaCl) x 100
= (23 / 58.5) x 100
= 39.32%
Mass % of Na = 39.32 %
Atomic mass of chlorine = 35.5 u
Mass % of Cl = (Atomic mass of Cl / Molecular mass of NaCl) x 100
= (35.5 / 58.5) x 100
= 60.68 %