Question

What is the pH of a 0.65 M solution of pyridine, C5H5N?

Answer 1

Hey there!

The pH of a weak base (in this case, pyridine is a weak base) is calculated by using the weak base equation, and then solving it.

C5H5N; Kb= 1.7 x 10⁻⁹

pH = -log [H⁺]

we can also say that pH = 14 + log [OH⁻]

Plugging into the weak base equation, we know that Kb = [OH⁻][BH⁺] / [B]

This can be solved using an ice chart, which will then simplify to :

Kb = [x][x] / [0.50-x]

We can assume that x is very small in comparison to the 0.5 , so we consider it zero.

Then plug in and solve for x.  1.79 *10-9 = x² / 0.5

x = 2.992 * 10⁻⁵

ince X is equal to the [OH⁻] concentration, we can plug it into the second pH equation, and get the pH!

pH = 14+ log 2.99 * 10⁻⁵

pH = 9.48

Hope that helps!