$pH$ of weak bases
Tutorial 9
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 $
Calculate the pH of a $ 0.010 \; M $ solution of cyclohexylamine $(pK_a=10.64) $
$pK_b\;=\;14.00-10.64\;=\;3.36$
$K_b=10^{-pK_b}=10^{-3.36}$
With $x=[OH^-]$, we have:
$x^2\;+\;10^{-3.36}\;x\;-\;10^{-3.36}\;0.010\;=\;0$
$x\;=\;2.09\cdot 10^{-3} $
$pOH\;=\;-log\; x\;=\;2.68$
$pH\;=\;14.00-2.68\;=\;11.32$
