Caffeine at equilibrium is 0.050 M with 5.0 x 10^-3 M conjugate acid and 2.5 x 10^-3 M hydroxide. What is Kb?

1. The base constant puts the conjugate acid times hydroxide over the unreacted base. 2. Multiply the two products: 5.0 x 10^-3 x 2.5 x 10^-3 = 1.25 x 10^-5. 3. Divide by the base concentration: 1.25 x 10^-5 divided by 0.050. 4. That gives 2.5 x 10^-4 as the base ionisation constant of caffeine. 5. Stopping at the numerator would give 1.25 x 10^-5, close to the first distractor. 6. Water is the solvent here, so its concentration never enters the expression. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.3 Relative Strengths of Acids and Bases_