Using pH to Determine Kb

Think About It Check your answer by using the calculated Kb to determine the pH of a 0.15 Absolution. 

Caffeine, the stimulant in coffee and tea, is a weak base that ionizes in water according to the equation C8HioN402(nq) + H20(/) HCgHioN402(aq) + OH (aq) 

Strategy Use pH to determine pOH, and pOH to determine the hydroxide ion concentration. From the hydroxide ion concentration, use reaction stoichiometry to determine the other equihbiium concentrations and plug those concentrations into the equilibrium expression to evaluate Kb- 

Based on the reaction stoichiometry, [HC8H10N4O2 ] = (OH ], and the amount of hydroxide ion in solution at equilibrium is equal to the amount of caffeine that has ionized. At equilibrium, therefore, 

Solution Plugging the equilibrium concentrations into the equilibrium expression gives [HC8HioN402](OH ] (2.82 X lO )  

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