[diane]

why is the Ka of Al(H20)6³⁺ used?

Isn't the reaction:
AlCl3 +  3H20 --> 3HCl + Al(OH)3 ?

[Borek]

For anhydrous AlCl₃ and small amounts of water yes, you can expect gaseous HCl as a product. But question asks about pH of the solution, not about a direct reaction. Solution can be prepared for example using hexahydrate.