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(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 xaxis. 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 ]
Reenter 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
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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.
 Spring '13
 PatrickLloyd
 Chemistry, Equilibrium, Bases

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