If the Kb of a weak base is 1.2×10−6, what is the pH of a 0.34 M solution of this base?

1 Answer
Truong-Son N.
May 4, 2017

It does not matter what base you are looking at. If you know its Kb and its concentration, that is enough. Denote a neutral base as B. Then the ICE table can be constructed.

B(aq) + H2O(l)⇌BH+(aq)+OH−(aq)

I 0.34 M − 0 M 0 M
C −x − +x +x
E (0.34−x)M − x x

Thus, the Kb is:

Kb=x20.34−x

In fact, the Kb is small enough to use the small x approximation. A good rule of thumb is to check whether K is on the order of 10−5. Hence, we have:

Kb≈x20.34

and the equilibrium concentration of OH− is readily obtained:

x=√0.34Kb

=6.39×10−4 M

Therefore, the pH is:

pH=14−pOH

=14−(−log[OH−])

=14−3.19

=10.81

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