Weak Bases Ammonia and the Amines

Ammonia is the simplest nitrogen-containing compound that acts as a weak base in water  

Despite labels on reagent bottles that read ammonium hydroxide, an aqueous solution of ammonia consists largely of unprotonated NH3 molecules, as its small Alb indicates. In a 1.0 M NH3 solution, for example, [OH ] = [NH4 ] = 4.2X10 M, so about 99.58% of the NH3 is not ionized. A list of several bases with their A b values appears in Appendix C. 

Finding the pH of a solution of a molecular weak base uses an approach very similar to that for a weak acid. We write the equilibrium expression, set up a reaction table to find [base]reacting make the usual assumptions, and then solve for [OH ]. The main difference is that we must convert [OH ] to [H30 ] in order to calculate pH. 

Plan We know the initial concentration (1.5 M) and Ky, (5.9X10 ) of (CH3)2NH and have to find the pH. The amine reacts with water to form OH , so we have to find [OH ] and then calculate [H3O ] and pH. The balanced equation and Ky expression are 

Because Ky, K, the [OH ] from the autoionization of water is negligible, and we disregard it. Therefore, 

As with the relation between pK and K y we know that pA b the negative logarithm of the base-dissociation constant, decreases with increasing (that is, increasing base strength). In aqueous solution, the two large classes of weak bases are (1) ammonia and the amines and (2) the anions of weak acids. 

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