(i) Formula of sodium oxide (made of sodium Na and oxygen O) :
Thus, the formula of sodium oxide is `Na_(2)O`
(ii) Formula of aluminium chloride (made of aluminium Al and chlorine Cl) :
Thus, the formula of aluminium chloride is `AlCl_(3)`.
(iii) Formula of sodium sulphide (made of sodium Na and sulphur S) :
Thus, the formula of sodium sulphide is `Na_(2)S`.
Formula of magnesium hydroxide (made of magnesium `Mg` and hydroxide `OH`) :
Thus, the formula of magnesium hydroxide is `Mg(OH)_(2)`