Step 1. Calculate the moles of #"FeSO"_4·7"H"_2"O"#
#"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"#
