Question

why do mass of carbon 12 is taken as standard atomic mass ?

Answer 1

Physicists picked the pure isotope Oxygen-16, because they tended tomake their measurements on the basis of mass spectrometry. ... So theAtomic Mass of Carbon-12 is defined to be 12 exactly and all other atomic, molecular and formula masses arereferred to this standard
Hope it will help you...

Answer 2

The mass values which we are using are relativeatomic masses when compared with 1/12 themass of carbon atom. Intially Hydrogen -1 massand Oxygen-16 are used as reference atomic masses. But they gave complications when used as reference to overcome this problem C- 12 hastaken as the standard

OR

Instead of using either Hydrogen, or Oxygen as the standard, the isotope of Carbon with 6 protons and 6 neutrons in its nucleus (Carbon-12) was given a massof exactly 12. ... Plus, Carbon-12's atomic mass could be measured particularly accurately compared to the other elements on the periodic table.