Chapter 19: Q15P (page 487)
Method of continuous variation. Make a graph of absorbance versus mole fraction of thiocyanate from the data in the table.
(c)Why does one solution containacid and the otheracid?
One solution contain acid and the other acid because ionic strength must be equal in both the solution.
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This problem can be worked by calculator or with the spreadsheet in Figure 19-4. Consider compounds X and Y in the example labeled “Analysis of a Mixture, Using Equations 19-6” on page 464. Find [X] and [Y] in a solution whose absorbance is 0.233 at 272 nm and 0.200 at 327 nm in a 0.100-cm cell.
Spectroscopic data for the indicators thymol blue (TB),semithymol blue (STB), and methylthymol blue (MTB) are shown in the table. A solution ofTB,STB,MTB in a1.000-cm cuvet had absorbances of 0.412at455nm,0.350 at485nm, and 0.632 at545nm. Modify the spreadsheet in Figure 19-4 to handle three simultaneous equations and findandin the mixture.
Find the concentration of [X] if the absorbance are 0.700 at 272 nm and
0.550 at 327 nm.
What is the advantage of a time-resolved emission measurement with Eu3+versus measurement of fluorescence from organic chromophores?
Two ways to analyze a mixture. Figure 19-5 shows the spectrum of the indicator bromothymol blue adjusted to several pH values. The spectrum at pHis that of the pure blue form and the spectrum at pH 1.8is that of the pure yellow form. At other pHvalues, there is a mixture of the two forms. The total concentration isand the path length isin all spectra. For the purpose of calculation, assume that there are more than two significant digits in concentration and path length. Absorbance at the dots on three of the curves in Figure 19-5 is given in the table.
(a) Prepare a spreadsheet like Figure 19-3 to use absorption at all six wavelengths to findin the mixture. Comment on the sum.
(b) Fromin the mixture, and fromfor HIn,calculate theof the mixture. (This calculation is the source of pH labels in the figure.)
(c) Use Equations 19-6 at the peak wavelengths ofto findin the mixture. Compare your answers to those in (a). Which answers, (a) or (c), are probably more accurate? Why?
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