$pH$ of weak bases

Tutorial 7

         

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 $

Dans le Q.C.M suivant, on abrège souvent $H_3O^+$ par $H^+$ (ce qui est faux en réalité, en effet, $H^+$ est toujours entièrement hydraté dans l'eau!)

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

Which of the following expressions represents K _b after substitution ?

a) $1.7 \cdot 10^{-4}$ $=$ $\frac{(0.050-x)x}{ x}$ b)$ 1.7 \cdot 10^{-4}$ $=$ $\frac{x^2 }{ 0.050}$ c) $1.7 \cdot 10^{-4}$ $=$ $\frac{0.050-x }{ x^2}$ d) $1.7 \cdot 10^{-4}$ $=$ $\frac{x^2 }{ 0.050-x}$