Question 11

11. Calculate the pH of a 0.050M solution of the weak base pyridine (

C

₅H₅N ) if it has

K

_b = 1.7 x

10

^-4.

Balanced equation: $C$ ₅H₅N (aq) + H₂O (l) HC₅H₅N⁺ (aq) + OH^- (aq)

I.C.E. table:

	$C$ ₅H₅N (aq)	$HC$ ₅H₅N⁺ (aq)	$OH$ ^- (aq)
CHANGE	-xM	+xM	+xM
EQUILIBRIUM	(0.050-x)M	xM	xM

K

_b = 1.7 x

10

^-4 = {x²} / {(0.050-x)}

Solving for x, we find that:

x =

9.2 x 10

^-6M = [OH

^-],
and thus:
[ H⁺ ] = {K_w} / {[ OH^- ]}

How do we find the pH?