To write the chemical formulae of compounds formed from ions, we use the crossover method where the valencies (charges) of the ions are crossed over to become subscripts. For (i) Fe³⁺ and OH⁻: The charges are 3+ and 1-. Crossing over gives Fe₁(OH)₃, which simplifies to Fe(OH)₃. Brackets are used around OH because there are three hydroxide ions (a polyatomic ion) bound to one iron ion. For (ii) K⁺ and CO₃²⁻: The charges are 1+ and 2-. Crossing over gives K₂(CO₃)₁, which simplifies to K₂CO₃. Brackets are not required when only one polyatomic anion is present in the formula.