Try This:

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1. Given the following reaction.

1. Write the equilibrium constant expression, K_a, for this reaction. Recall that the ammonium ion (NH₄⁺) is the conjugate acid for aqueous ammonia.
   
   
2. 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.)
   
   
3. Using the equilibrium concentrations that are determined by the Virtual Lab Simulator for the 0.01 M NH₄Cl solution, calculate a value for K_a and pK_a. Compare these values to the textbook values shown above in Figure 3.

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1. ,
   
   The H₂O is not included in this equation because it is the solvent and its concentration is unaffected by the reaction.
   
   
2. 
   
   
   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.
   
   
3. 
   ;
   
   These values agree quite well with those shown in Figure 3 for NH₄⁺

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