#16# grams of Oxygen atom = #1# mole
#1# mole = #6.023# x #10^23# molecules
#16# grams of Oxygen atom have molecules = #6.023# x #10^23# molecules
#0.25# g of Oxygen atom will have molecules = (#6.023# x #10^23# molecules) x #0.25# g]/#16# g = #9.41# x #10^21# molecules
OR
Oxygen = #0.25# g
molar mass = #16# g
no. of moles = #0.25/16# = 0.0156 moles
#1# mole = #6.023# x #10^23# molecules
so, #0.0156# moles of Oxygen atom = #0.0156# moles x #6.023# x #10^23# molecules = #9.39# x #10^21# molecules = #9.4# x #10^21# molecules