Question

What is the pH at 25 degrees Celsius of a 0.0064 M solution of a weak base with a Kb of 1.6E-9?

Answer 1

pH = 8.5

Explanation:

A weak base in water steals a proton off water to make hydroxide ion, and from this you can get your pH.

          B: + H20 = BH + OH-

I 0.0064M 0 0
C -x +x +x
E 0.0064M +x +x

employ the x<<0.0064M assumption to avoid quadratic, then check at end to make sure it is less than 5%

`Kb=1.6xx10^-9 = "[BH][OH]"/"[B:]"`
`Kb=1.6xx10^-9 = "[x][x]"/"[0.0064M]"`
`x = 3.2xx10^-6`.....`(3.2xx10^-6)/0.064xx100 = 0.05%` assumption is ok

`x = 3.2xx10^-6` pOH = 5.5 pH = 8.5