How many moles of O are in 10.8mol Fe(NO3)3?

There are ##"97.2 mol of O"## in ##"10.8 mol of Fe"("NO"_3)_3##.

There are 9 atoms of ##"O"## in 1 of ##"Fe"("NO"_3)_3##, so there are ##"9 mol of O"## in ##"1 mol of Fe"("NO"_3)_3##.

Your conversion factor is either ##"9 mol O"/("1 mol Fe"("NO"_3")_3)## or ##("1 mol Fe"("NO"_3)_3)/"9 mol O"##.

You choose the one that cancels units to give the desired units on the answer.

##10.8 color(red)(cancel(color(black)("mol Fe"("NO"_3)_3))) × "9 mol O"/(1 color(red)(cancel(color(black)("mol Fe"("NO"_3)_3)))) = "97.2 mol O"##