Question

how to calculateand learn mass number in chemistry

Answer 1

Mass number is equal to the sum of the number of the protons and neutrons in the nucleus.
You need not memorize the mass numbers of all elements in the periodic table. Just knowing the mass nos. of hydrogen, helium, lithum, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, chlorine, copper, iron, gold and silver will suffice.