To answer this question, we will need to write the dissociation equation of aluminum trichloride.
AlCl3 ——-> Al3+ + 3Cl-
It can be seen that when aluminum chloride dissociates, it gives one mole of aluminum ion and three moles of the chloride ion.
From here we can see that the concentration of the aluminum chloride equals that of the aluminum ion while that of the chloride ion is thrice that of the aluminum chloride. This means we simply multiply 0.12M by 3 to get the molarity of the chloride ion while that of the aluminum ion remains the same
