In 2.6 kg of FeSO₄.6H₂O (atomic weight of Fe=56 g/mol):

Mass of FeSO₄.6H₂O is 2.6 kg or 2600 g.  FeSO₄.6H₂O has 12 H atoms, 10 O atoms, 1 Fe and 1 S atom.

Moles of  FeSO₄.6H₂O:

$\mathrm{Moles}=\frac{2600 \mathrm{g}}{260 \mathrm{g}/\mathrm{mol}}=10 \mathrm{mol}$

Number of O atoms in FeSO₄.6H₂O:

$10\times \mathrm{n}\times {\mathrm{N}}_{\mathrm{A}}=10\times 10\times {\mathrm{N}}_{\mathrm{A}}=100{\mathrm{N}}_{\mathrm{A}}$

Number of H atoms in FeSO₄.6H₂O:

$12\times \mathrm{n}\times {\mathrm{N}}_{\mathrm{A}}=12\times 10\times {\mathrm{N}}_{\mathrm{A}}=120{\mathrm{N}}_{\mathrm{A}}$

Number of H₂O moles in FeSO₄.6H₂O:

$6\times \mathrm{n}\times {\mathrm{N}}_{\mathrm{A}}=6\times 10\times {\mathrm{N}}_{\mathrm{A}}=60{\mathrm{N}}_{\mathrm{A}}$

Number of ${\mathrm{SO}}_4^{2-}$ moles in FeSO₄.6H₂O:

$1\times \mathrm{n}\times {\mathrm{N}}_{\mathrm{A}}=1\times 10\times {\mathrm{N}}_{\mathrm{A}}=10{\mathrm{N}}_{\mathrm{A}}$