Question

Use the pH measurement you made for each NH3 solution to calculate a value of Kb and pKb for NH3.

Beaker #	pH measured	Concentration
1	15.7	1.00M
2	14.4	0.10M
3	13.7	0.01M

It seems that I keep getting a negative Kb or pKb value which can not be correct.

Answer 1

Let α be the dissociation of the weak base
NH₃ + H₂O <---> NH₄⁺ + OH^-

initial conc.            c 0         0

change               -cα +cα      +cα

Equb. conc.         c(1-α) cα      cα

Dissociation constant, K_b = (cα x cα) / ( c(1-α)               

                                          = c α² / (1-α)

In the case of weak bases α is very small so 1-α is taken as 1

So K_b = cα²

Given concentration ,c = 0.01 M

pH = 13.7

pOH = 14 - pH = 14-13.7 = 0.3

- log [OH^-] = 0.3

[OH^-] = 10^-0.3 = 0.5012M

cα = 0.05012 M

Also c = 0.01 M

So α = cα / c = 50.12

K_b = cα²

   = 0.01 x (50.10)2

= 25.12

pKb = - log Kb

= - log 25.12

= -1.4

The negative value is obtained because the measured pH values be more than 14