The hydrated aluminium ion has Ka = 1.4 x 10^-5. What is the pH of 0.10 M aluminium chloride?

1. Aluminium chloride dissolves completely to give the hydrated aluminium ion. 2. That ion acts as a weak acid, so this is an ordinary weak acid calculation. 3. Let x be the hydronium concentration formed from a start of 0.10 M. 4. Assume x is small: x squared = 0.10 x 1.4 x 10^-5 = 1.4 x 10^-6. 5. Take the square root: x = 1.2 x 10^-3 mol per litre. 6. That is about 1 percent of 0.10, so the assumption is sound. 7. Take the negative logarithm: pH = 2.92, which is clearly acidic. 8. Treating the salt as a strong acid would give pH 1.00, the first distractor. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.4 Hydrolysis of Salt Solutions_