Question

how can we caculate atomic mass when mass of a single atom of an element is given

Answer 1

To calculate the mass of a single atom, first look up the atomic mass of carbon from the Periodic Table.
This number, 12.01, is the mass in grams of one mole of carbon. One mole of carbon is 6.022 x 1023 atoms of carbon (Avogadro's number). This relation is then used to 'convert' a carbon atom to grams by the ratio:

mass of 1 atom / 1 atom = mass of a mole of atoms / 6.022 x 1023 atoms

Plug in the atomic mass of carbon to solve for the mass of 1 atom:

mass of 1 atom = mass of a mole of atoms / 6.022 x 1023

mass of 1 C atom = 12.01 g / 6.022 x 1023 C atoms
mass of 1 C atom = 1.994 x 10-23 g