Unknown Indicators, Acids, and Buffers
If we knew what it was we were doing, it would not be called research, would it? – Albert Einstein In the following three experiments, you will design your own experiments. We will explain the general problem — the analysis of an unknown — and give you a brief introduction to any necessary theory. Working in a team of students, you will design and carry out every aspect of the experiment. You and your team will have to answer many seemingly mundane questions for yourself. How much solution should you make? What concentration should the solution be? What glassware should you use? You will also have to come up with your own strategy. You will have a total of 5 lab periods to complete 3 experiments: the pKa of an unknown indicator, the identity of an unknown solid acid, and the composition of an unknown acetic acid buffer. We strongly suggest that you do not try to leave early until your group has finished all of the experiments and analyzed all of your data. Do not fall behind, as you will not be given additional time. The suggested timeline is as follows: Week 1: pKa of Unknown Indicator Week 2: Unknown Acid Week 3: Unknown Acid Week 4: Unknown buffer solution. Week 5: Unknown buffer solution I have one very important suggestion: Think before you act! The pre-laboratory questions for these experiments are designed to get you thinking along the right track. Because of time constraints, you must have a clear picture of what you are doing in your experiment before you start. Spending an extra 10 minutes discussing your plans with your group can easily save hours in the lab. Before you take your data, make sure that you know exactly how you will analyze your results. Otherwise, you might forget to make some vital measurements. Also, make sure to record all your observations in your notebook. Most importantly, have fun. This is what chemistry is all about. — MAH
Unknown Acids, Indicators, and Buffers
The pKa of an Unknown Acid-Base Indicator
Your group will be assigned a solution containing an unknown amount of an unknown acid-base indicator. In the following, you will design and carry out a series of experiments designed to first qualitatively and then quantitatively measure the pKa of this indicator dye, which is defined to be pK a = − log K a .
This definition is in direct analogy to pH, which is defined to be
pH = − log ⎡ H + ⎤ . ⎣ ⎦
Each group will have access to one pH meter and one visible spectrometer. Because of this, it is important that you work efficiently when making your measurements. After your team decides on an experimental strategy, everyone should be making measurements. For this reason, it is important that you operate as a team in scheduling and sharing equipment. At the end of the experiment, you will share your results with your team members; however, part of your grade will be determined by whether you performed your fair share of the team’s Fig. 1: A color wheel summarizes the experiments. relationship between the perceived An indicator dye is just a weak acid (or base) that changes color color of a substance and the color of when it loses (or gains) a proton. For the weak acid case, this light that it absorbs, which is the process can be qualitatively described by the reaction complementary color. The + – approximate wavelengths of the colors (3) HIn H + In Kc = Ka Color 1 Color 2 are also listed. A =λ A =λ max 1 max 2
For example, the popular indicator bromthymol blue is yellow at low pH (high [H+]), turning blue-green at pH ~ pKa ~ 7.0 before becoming dark blue at high pH (low [H+]). This behavior is shown in Fig. 21.7 in McQuarrie et al. The visible color of a solution can be quantified by measuring the colors of light that solution absorbs, or its absorbance, using a visible spectrometer. For example, if a solution appears yellow in white light (i.e. normal room light), the solution must be selectively absorbing...
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