A solution has 0.050 M acetate and 2.5 x 10^-6 M hydroxide, with Kb = 5.6 x 10^-10. What is the acetic acid concentration?

1. The acetate ion acts as a base here, so hydroxide is one of the products. 2. Its base constant is Kw over the acid constant, which is 5.6 x 10^-10. 3. The expression puts acetic acid times hydroxide over acetate. 4. Rearrange: acetic acid equals Kb times acetate, divided by hydroxide. 5. Numerator: 5.6 x 10^-10 x 0.050 = 2.8 x 10^-11. 6. Divide by the hydroxide concentration: 2.8 x 10^-11 divided by 2.5 x 10^-6. 7. That gives 1.1 x 10^-5 mol per litre of acetic acid. 8. Stopping at the numerator, 2.8 x 10^-11, is the trap of skipping the final division. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.4 Hydrolysis of Salt Solutions_