Trimethylamine has Kb = 6.3 x 10^-5. What is the pH of a 0.25 M solution of it?

1. Let x be the hydroxide concentration formed, so the amine sits at 0.25 minus x. 2. The statement is 6.3 x 10^-5 = x squared over (0.25 minus x). 3. Assume x is small against 0.25, giving x squared = 0.25 x 6.3 x 10^-5 = 1.575 x 10^-5. 4. Take the square root: x = 4.0 x 10^-3 mol per litre of hydroxide. 5. That is under 5 percent of 0.25, so the assumption is justified. 6. Take the negative logarithm of the hydroxide value: pOH = 2.40. 7. Convert with the sum rule: pH = 14.00 minus 2.40 = 11.60. 8. Quoting 2.40 is the trap of reporting the pOH as though it were the pH. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.3 Relative Strengths of Acids and Bases_