CHM12 Experiment 6 Determination of the Equilibrium Constant for Bromocresol Green

Dissociates to give b2 hb aq h2o l h3o aq b2 aq

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Unformatted text preview: partially dissociates to give B–2: HB– (aq) + H2O (l) H3O+ (aq) + B2– (aq) Writing the dissociation without water (shorthand notation), we have HB – ( aq ) H + (aq) + B2- (aq) K= basicform (blue) acidicform (yellow) [H + ][B2- ] [HB- ] HB– is the acidic form and is yellow in solution. B2– is the basic form and is blue in solution. Taking logarithms of the above equation gives log K = log [H + ]+ log log y [B2- ] [HB- ] [B2- ] = pH + log K [HB- ] =mx + b where we have rearranged and noted that pH = –log [H+]. [B2- ] versus pH should yield a straight line with a slope of 1 (m [HB- ] = 1) and an intercept equal to log K, where K is a concentration equilibrium constant. So our strategy [B2- ] will be to measure the ratio log as a function of pH and use this data to determine the equilibrium [HB- ] constant for the dissociation of bromocresol green. This is a linear equation. A plot of log 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 2 Absorbance and Spectrophotometry Solutions that possess colors absorb visible light energy of specific wavelengths. Recall that a red solution appears red because it absorbs much of the blue-green part of the spectrum (complementary colors). Measurements of the amount of light absorbed by a substance at each wavelength (color) can be graphed giving an “absorption curve.” The shape of this curve depends almost entirely on the electronic structure of the substance and is almost unique for each substance. Thus the curve serves as an aid to identification and, with the aid of modern theory, a clue to the structure of a substance. At a given wavelength the amount of light absorbed by a solute is proportional to its molar concentration, thus providing a widely used method of concentration analysis. The Beer-Lambert Law states that A = εlc, where A = absorbance, ε = a constant characteristic of the absorbing molecule, l = path length, c = concentration. In our case, ε and l are each constant (known absorbing substance and a path length determined by the width of the cuvette). Thus the absorbance is proportional to the concentr...
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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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