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Chemistry LibreTexts

Qual Analysis of Unknown Crystals

  • Page ID
    150563
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    INTRODUCTION

    Test tube rack with multiple test tubes showing different reaction products.

    This is the setup for the experiment

    Qualitative analysis is built in large part on separations. You must be able to separate components of a solution so that they can be identified. In this experiment, you will use the table of behavior of ions in acids and bases in the appendix of your lab notebook to determine a reasonable hypothesis for what could be in your unknown crystals.

    First, you will complete "quick tests" that mimic the organization of the table of behaviors of ions in your lab manual. From this, you will determine a reasonable flow chart for testing the possible ions (alone, in a group, or as a class). Be careful that you record all of the observations that this table guides you to record. This is good practice for recording observations for any qualitative tests.

    The table for "quick tests" is in your lab manual

    After verifying your flow chart, you will perform this new qual scheme on your unknown crystals (and a known). This lab will be completed over multiple weeks.

    image (1).png

    The table is in your lab manual.

    You will be able to determine what your unknown crystals may contain after completing this chart. Fully understanding and utilizing this chart. once completed, will help you in constructing your own flow chart in Q1-3.

     


    1.1: Slightly Acidic: xs 6M HCl

    By ensuring the pH of a solution is slightly acidic (pH around 5 or 6), all ions should be in solution. Colors can be quite misleading, so do not rule out anything based on the color you perceive (especially not the colorless ions!)

     


    1.2: Neutral pH ~ 7: HOAc/NH3 Buffer

    A selection of ions precipitate at a pH around neutral (pH = 7). It is difficult to get a pH exactly at 7 by using strong acids and strong bases. Instead, we will add a weak acid, acetic acid (HOAc (aq), HC2H3O2 (aq), C2H3OOH (aq) ), until the solution is barely acidic (just below pH = 7) and then add a weak base, ammonia (NH3 (aq) ) until the solution is neutral.

     


    1.3: Excess base: xs 6M NaOH

    In highly basic solution, many ions form precipitates. To see what (if any) ions may be present, add excess NaOH until the solution is definitely basic.

     


    1.4: Excess ammonia: xs 6 M NH3

    Many ions form a complex with ammonia. This changes the color of the solution. Since ammonia is a weak base, some ions will precipitate out with as a hydroxide.

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