Textbook 6D.15b

[Nancy Li 1C]

Can anyone explain how to do part b of this question? I'm not sure what the base/acid is and its protonation reaction

[Henry_2A]

Hi! 0.055 M AlCl3 is actually a salt so break it down to its respective cation and anion. Cl- is neutral while Al3+ is acidic; therefore, the solution will be acidic. Given ka of Al3+ = 1.4*10^-5, we can set that equal to x^2/(0.055) (omitted the 'x' in 0.055-x) because it's effect is negligible. Solve for x, and take -log[x] to get the pH.

I hope this helps!