[tamubsa]

The question is will the pH increase, decrease, or stay the same when you add solid ammonium chloride to 75 mL of 0.016 M HCl? The answer is that it will decrease. I understand that ammonium chloride is an acidic salt, and I think that is why the pH will decrease, but why wouldn't the chloride reduce the dissociation of HCl (common ion effect) and therefore increase the pH?

[Borek]

HCl is so strong you can safely ignore common acid effect. Especially in solutions so dilute.

[Big-Daddy]

It's a battle between the basic action of the added Cl^- (Cl^- + H₂O  HCl + OH^-) and the acidic action of the added NH₄⁺ (NH₄⁺  H⁺ + NH₃). The equilibrium constant for the first reaction is K_w/K_a(HCl) which is going to be tiny; the equilibrium constant for the second reaction is K_w/K_b(NH₃) which is pretty small, about 6*10^-10, but many orders of magnitude larger than the equilibrium constant for the first. So acidic action outweighs basic action and the [H⁺] increases so pH drops.

(Room temperature = 25°C assumed  )