CHM12 Experiment 6 Determination of the Equilibrium Constant for Bromocresol Green

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Unformatted text preview: tate* (mol) moles sodium acetate moles acetic acid mol NaAc log mol HAc pKa of HAc 4.74 4.00 mL 4.74 6.00 mL 4.74 8.00 mL 4.74 *From 1. Department of Physical Sciences Kingsborough Community College The City University of New York Spring 2012 Experiment 6: Determination of the Equilibrium Constant for Bromocresol Green 10 pH 4: Equilibrium Constant K To find the equilibrium constant (Ka) of bromocresol green we need to perform a plot of: [B2- ] versus pH where pH is the x-axis. The following table shows the progression needed to find log [HB- ] values needed for this graph. As a reminder (see page 3 for a more detailed discussion), AII is the absorbance of the basic form of bromocresol green at the wavelength of maximum absorbance (λmax). This absorbance was determined in Part 1 of the experiment and is a constant. AI is the absorbance of the acidic form of bromocresol green at the wavelength of maximum absorbance (λmax). This absorbance was also determined in Part 1 of the experiment and is also a constant. AX is the absorbance of the bromocresol green solution after 2.00 mL additions of acetic acid solutions were made. With each addition of acetic acid solution, the bromocresol green became slightly more dilute, causing its absorbance to decrease. To compensate for this dilution use the following equation to correct for the dilution. Enter the values of these corrections in the column “Corrected AX.” 100 mL + Vadded Corrected Absorbance = A X 100 mL . Note that, unlike AII and AI, Corrected AX varies. To complete the table, take the difference between Corrected AX and AI (Ax – AI), the difference between AII [B2- ] and AX (AII – Ax), and then take the ratio of these differences. The log of this ratio is equal to log . [HB- ] Re-enter the pH values from Step 3 in the calculations. Once you have completed the table you can construct the graph of log AII AI [B2- ] versus pH and find Ka. [HB- ] Corrected Ax A x -A I A II -A x log A x -A I [B2- ] = log A II -A x [HB- ] pH Department of Physical Sciences Kingsborough Community College The City University of New York Spring 2012 Experiment 6: Determination of the Equilibrium Constant for Bromocresol Green 11 Report 1. Hand in your origin...
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This note was uploaded on 02/11/2014 for the course CHEM 12 taught by Professor Patricklloyd during the Spring '13 term at CUNY Kingsborough Community College.

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