Acids, Bases, and Buffers 2: Unknown Solutions Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Appendix Experiments Index ChemLab Home		Prelab Problems 1. Consider the weak base ammonia: a. Write the reaction between NH3 and water and the equilibrium constant expression. b. What is the conjugate acid of NH3? What is the pKb of NH3 and what is the pKa of its conjugate acid? c. What is the pH of a 0.1 M solution of NH3? d. Calculate the pH at the equivalence point when 10.0 mL of 0.10 M NH3 is titrated with 0.1 M HCl. How can the pKa of NH3 be determined experimentally from a titration graph of pH vs. volume of titrant added? e. If you wanted to determine the concentration of NH3(aq) by titration, what indicator, phenolphthalein, methyl red or cresol red, would you use and why? (Use the information in the Appendix: Introduction to Acid/Base Indicators at the end of the experiment in the manual on pp. 163 - 164. 2. You obtain your unknown solution and determine that it has a pH of 9.25. What conclusions can you reach about its composition? What further experiments will you need to perform to characterize it? Be specific. 3. You obtain your unknown solution and determine that it has a pH of 2.9. You decide it is not a buffer from the results of titration with acid and base. What further experiments will you need to perform to identify which weak acid your sample contains? Be specific.
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