Question

Calculate the concentration of pyridine, C5H5N, in a solution that is 0.70 M pyridinium bromide, C5H5NHBr. (Assume Kw is 1.0 10-14.) What is the pH of the solution?

Answer 1

SOLUTION:

Given that, Concentration of pyridinium bromide, [C₅H₅NHBr] = 0.70 M

From ICE table:

	C5H5NH+(aq)	C5H5N (aq)	H3O+(aq)
Initial (M)	0.7	0	0
Change (M)	-x	+x	+x
End (M)	0.7-x	x	x

We have,

K_b for C₅H₅N = 1.77 x 10^-9

Also We have, K_a x K_b = K_w

K_a x 1.77 x 10^-9 = 1.0 x 10^-14

K_a = ( 1.0 x 10^-14)/(1.77 x 10^-9 )

K_a = 5.6497 x 10^-6

Now, K_a expression is given by,

(since, x <<< 0.7) =>

That is:

Therefore, pH is given by: