To calculate the number of oxygen atoms in 0.800 moles of MgO, we need to know the chemical formula of magnesium oxide (MgO) and the Avogadro's number (Nₐ = 6.022 × 10^23 atoms/mol).

The chemical formula MgO indicates that one molecule of magnesium oxide contains one magnesium atom (Mg) and one oxygen atom (O).

So, in 0.800 moles of MgO, we can calculate the total number of oxygen atoms as follows:

Number of oxygen atoms = (number of moles of MgO) × (number of O atoms per molecule of MgO) × (Avogadro's number)

Number of oxygen atoms = (0.800 moles) × (1 O atom/molecule) × (6.022 × 10^23 atoms/mol)

Number of oxygen atoms ≈ 4.818 × 10^23 atoms

Therefore, there are approximately 4.818 × 10^23 oxygen atoms in 0.800 moles of MgO.