To figure this out, you need to figure out how many molecules of mgso4 you have.
If you have 1.04 moles of MgSO4, you have that many moles times avogadro's number (6.022*10^23 molecules/mole).
1.04 moles MgSO4 * (6.022*10^23 molecules MgSO4)/1 mole MgSO4 =
6.263 *10^23 molecules MgSO4
Next, there are 4 oxygen atoms per molecule of MgSO4, so we multiply the number of molecules of MgSO4 by 4 to get the number of oxygen atoms.
6.263 *10^23 molecules MgSO4 * (4 oxygen atoms/1 MgSO4 molecule) = 2.51 * 10^24 oxygen atoms in 1.04 moles of MgSO4
