$pH$ of weak bases

Tutorial 8

         

Calculation of the pH of a weak base List of weak acid / base couples You can find here the list of weak acid / base pairs with their $ pK_a $ Method for calculating the pH of a weak base solution $ B $ 1) Search the $ pK_b $: $ pK_b = 14.0-pK_a $ 2) Find the basicity constant $ K_b $: $ K_b = 10^{-pK_b} $ 3) Set $ x = [OH^-] $ and then solve the equation of the second degree: $ x^2 \; + \; K_b \; x \; - \; Kb \; c_{B} \; = \; 0$ Where $ c_{B} $ is the initial concentration (before dissociation) of $ B $ 3) Calculate: $ pOH = -log \; x $ $ pH = 14.0-pOH $

Here is a $ 0.050 \; M $ solution of the weak base pyridine $(C_5H_5N$ , $K_b$ $=$ $1.7\cdot 10^{-4})$ $C_5H_5N$ $ +$ $H_2O$ $HC_5H_5N^+$ $+$ $OH^-$

We found: $[OH^-]$ $=$ $x$ $=$ $1,3\cdot 10^{-2} \;M$ How to calculate the $ pH $ of this solution ?

a) find $pH$ $=$ $- log [ H_3O^+ ]$ b) find $pOH$ $=$ $- log[ OH^- ]$, then $pH$ $=$ $14-pOH$ c) find $[ H_3O^+ ]$ $=$ $\frac{K_e}{[ OH^- ]} $ , then $pH$ $=$ $- log [ H_3O^+ ]$ d) find $[ H_3O^+ ]$ $=$ $10^{-pH}$