An acid-base indicator is usually a weak acid with a characteristic color in the protonated (acid) and deprotonated (conjugate base) forms. In this assignment, you will monitor the color of an acetic acid solution containing Bromocresol Green as an indicator, as the pH is changed and then you will use your data to calculate the ionization constant, Ka, for the bromocresol green indicator and compare to an accepted value. 1. Start Virtual Chemlab, select Acid-Base Chemistry, and then select lonization Constants of Weak Acids from the list of assignments. The lab will open in the Titration laboratory. Bottles of 0.1104 M NaOH and 0.1031 M HAC (acetic acid) will be on the lab bench. The buret will be filled with the NaOH solution and a beaker containing 10.00 mL of the HAc solution will be on the stir plate. The stir plate will be on, Bromocresol Green indicator will have been added to the beaker, and a calibrated pH probe will also be in the beaker so the pH of the solution can be monitored.
2. What is the color and pH of the solution?
3. On the buret, the horizontal position of the orange handle is off for the stopcock. Open the stopcock by pulling down on the orange handle. The vertical position delivers solution the fastest with three intermediate rates in between (slow dropwise, fast dropwise, and slow stream). Turn the stopcock to the second position or fast drop-wise addition. Observe the color of the solution and close the stopcock when you observe the change in color by double clicking on the center of the stopcock.
4. What is the color and pH of the solution now?
5. Continue to add NaOH as before or at a faster rate. What is the final color and pH of the solution after all of the NaOH is added?
6. An acid-base indicator is usually a weak acid with a characteristic color in the protonated and deprotonated forms. Because bromocresol green is an acid, it is convenient to represent its rather complex formula as HBOG. HBOG ionizes in water according to the following equation:
HBOG + H2O = BCG + H3O+
(yellow) (blue)
The K. (the equilibrium constant for the acid) expression is:
Ka = [BCG-][H3O+]/[HBCG)
When [BCG-[ = [HBCG), then Ka = [H3O+). If you know the pH of the solution, then the [H3O+] and Ka can be determined.
What would be the color of the solution if there were equal concentrations of HBCG and BCG-?
What is the pH at the first appearance of this color?
What is an estimate for the Ka for bromocresol green?