Determination of Equilibrium Constant and Pka Through Spectrophotometry

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The swim bladder of a fish contracts and expands, allowing freedom to swim without floating or sinking abruptly; this is equilibrium in its finest. Equilibrium is a critical part of everyday life; in the field of chemistry, it specifically refers to the moment when the rates of both the forward and reverse reaction are equal. [2] In this particular experiment, spectrophotometry was used to measure absorbance of a buffer, an acid, and a base. Due to the basic rules of equilibrium, the concentrations were able to be determined, which could then be utilized to find the equilibrium constant and its pKa for bromothymol blue. First, 0.4040 grams of NaH2PO4 and 0.3989 grams of Na2HPO4 were added to a 150 mL beaker, along with 50 mL of distilled water. Next, 20 drops of bromothymol blue were inserted into the solution. The program LoggerPro and a Vernier pH probe were used to calculate the pH. Then, a volumetric flask was used to measure 5 mL of the buffer solution into each of three, separate 50 mL beakers. After rinsing a volumetric pipet with 1.0 M HCl, 1.00 mL of 1.0 M HCl was transferred to one of the three beakers. Next, the pipet was rinsed with distilled water and with 1.0 M NaOH solution. The pipet was then used to put 1.00 mL of 1.0 M NaOH into a second beaker. Finally, the pipet was once again rinsed once with distilled water, and 1.00 mL of distilled water were added to the third beaker. The pH and temperature probe were used to record the pH and temperature of each of these three solutions. After obtaining four cuvettes, one was filled with distilled water, and the other three were each filled with one of the three solutions prepared earlier. First, the spectrophotometer was adjusted to 0%. Then the blank was inserted, and the reading was set to 100%. The transmittance % was then determined for each of the three colors at 360 nm. The recordings were

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