Step 1. Calculate the moles of "FeSO"_4·7"H"_2"O"FeSO4⋅7H2O
"Moles of FeSO"_4·7"H"_2"O" = 278 color(red)(cancel(color(black)("g FeSO"_4·7"H"_2"O"))) × ("1 mol FeSO"_4·7"H"_2"O")/(278 color(red)(cancel(color(black)("g FeSO"_4·7"H"_2"O")))) = "1.000 mol FeSO"_4·7"H"_2"O"
Step 2. Calculate the moles of "O" atoms
"Moles of O atoms" = 1.000 color(red)(cancel(color(black)("mol FeSO"_4·7"H"_2"O"))) × "11 mol O atoms"/(1 color(red)(cancel(color(black)("mol FeSO"_4·7"H"_2"O")))) = "11.00 mol O atoms"
Step 3. Calculate the number of "O" atoms
"Number of O atoms" = 11.00 color(red)(cancel(color(black)("mol O atoms"))) × (6.022 × 10^23 "O atoms")/(1 color(red)(cancel(color(black)("mol O atoms"))))
= 6.62 × 10^24 color(white)(l)"O atoms"