(i) Sodium hydrogencarbonate: The context explicitly mentions its formula as NaHCO₃. It is formed by sodium (Na⁺) and hydrogencarbonate (HCO₃⁻) ions.

(ii) Sulfur dioxide: It is a covalent compound formed by one sulfur atom and two oxygen atoms, making the formula SO₂.

(iii) Ferric chloride: The term 'ferric' represents the iron ion with a +3 charge (Fe³⁺), and 'chloride' represents the chlorine ion with a -1 charge (Cl⁻). Balancing the +3 charge of iron with three -1 charges of chlorine yields the formula FeCl₃.

(iv) Cuprous oxide: The term 'cuprous' represents the copper ion with a +1 charge (Cu⁺), and 'oxide' represents the oxygen ion with a -2 charge (O²⁻). To balance the -2 charge of oxide, two cuprous ions are needed, resulting in the formula Cu₂O.