Write the equilibrium constant expression, Ka, for this reaction. Recall that the ammonium ion (NH4+) is the conjugate acid for aqueous ammonia.
Use the Virtual Lab Simulator, load the Unit 6 homework (File > Load Homework > Unit 6 > Elaborations - Definitions of Acids and Bases), and determine the pH of a 0.01 M solution of NH4Cl. Is NH4+ ion behaving like an acid as predicted by this equation?
(Note: Because NH4+ is ionic, to make up a 0.01 M solution of NH4+, you need to make up a solution of an ionic compound that contains NH4+. Ammonium chloride (NH4Cl) is a good choice of an ionic compound for doing this.)
Using the equilibrium concentrations that are determined by the Virtual Lab Simulator for the 0.01 M NH4Cl solution, calculate a value for Ka and pKa. Compare these values to the textbook values shown above in Figure 3.
The H2O is not included in this equation because it is the solvent and its concentration is unaffected by the reaction.
The pH is 5.62. Yes, the NH4+ ion is behaving like an acid because it lowered the pH of the solution from 7.0 to 5.62.
These values agree quite well with those shown in Figure 3 for NH4+