Hey there!:
1 mole of FeSO4 = 151.8 g/mol
25.0 g change to mole = 25.0 / 151.8
moles of FeSO4 = 0.165 moles
One mole of any substance has 6.02*10²³ units
0.165 moles to get the number of Fe ( II ) ions :
number of Fe(II) ion = 0.165 * ( 6.02*10²³ )
number of Fe ( II ) ion = 9.93*10²² units