The mass in grams of 2.75 x10^21 is calculated as
by use of avogadros law
1mole = 6.02 x 10 ^23 atoms
what of 2.75 x 10^23 atom
2.75 x 10^21atoms x 1mole/ 6.02 x 10^23= 4.568 x10 ^-3moles
mass is therefore = moles x molar mass of lithium(6.941)
that is (4.568 x 10^-3) x 6.941 =0.0317 grams