Answer: 4
Justification:
1) The dissolution of a formula unit of AlCl₃ is given by the chemical equation:
AlCl₃ (s) → Al₃⁺ + 3 Cl⁻
So, 1 unit of AlCl₃ yields 1 partilce of Al₃⁺ and 3 particles of Cl⁻.
2) You get that through a mass balance and the oxidation states (balance of charge).
i) Mass balance: the same number of atoms of each species must appear on the reactant and the product sides.
ii) The oxidation state of Al is 3+ and the oxidation state of Cl is 1-, so the total net charge is 3+ + 3(1-) = 3 - 3 = 0.