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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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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.
Simulating a Job鈥檚 plot. Consider the reaction . Suppose that the following mixtures of A and B at a fixed total concentration of
10-4 M are prepared:
(b)Prepare a graph by the method of continuous variation in which you plotversus mole fraction offor each equilibrium constant. Explain the shapes of the curves.
Explain what is done in flow injection analysis and sequential injection. What is the principal difference between the two techniques? Which one is called 鈥渓ab-on-a-valve鈥�?
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?
A study was conducted with derivatives of the DNA nucleotide bases adenine and thymine bound inside micelles () in aqueous solution.
Sodium dodecyl sulfate forms micelles with the hydrocarbon tails pointed inward and ionic headgroups exposed to water. It was hypothesized that the bases would form ahydrogen-bonded complex inside the micelle as they do in DNA:
To test the hypothesis, aliquots of 5.0 mMadenine derivative were mixed with aliquots of 5.0 mMthymine derivative in proportions shown in the table. Each solution also contained 20mMsodium dodecyl sulfate. The concentration of product measured by nuclear magnetic resonance also is shown in the table. Are the results consistent with formation of a 1:1complex? Explain your answer.
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