Chapter 19: Q19P (page 488)
Explain what is done in flow injection analysis and sequential injection. What is the principal difference between the two techniques? Which one is called “lab-on-a-valve”?
It is called “ lab-on- a valve because sample, reagents, and product all flow through a central multiport valve.
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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.
Infrared spectra are customarily recorded on a transmittance scale so that weak and strong bands can be displayed on the same scale. The region near in the infrared spectra of compounds A and B is shown in the figure. Note that absorption corresponds to a downward peak on this scale. The spectra were recorded from a 0.0100M solution of each, in cells with 0.00500 - cm path lengths. A mixture of A and B in a 0.00500 - cm cell gave a transmittance 34.0 % of at2022cm and 383% at 1093 cm. Find the concentrations of A and B.
Now we use Solver to find Kfor the previous problem. The only absorbing species at 332 nmis the complex, so, from Beer’s law is either free or bound in the complex,so There is a huge excess of mesitylene, so
The spreadsheet shows some of the data. You will need to use all the data. Column A contains [mesitylene] and column B contains . Column C lists the measured absorbance. Guessa value of the molar absorptivity of the complex,Then compute the concentration of the complex in column D. The equilibrium constant in column
should we minimize with Solver? We want to varyεin cell A7 until the values of Kin column E are as constant as possible. We would like to minimize a function like , where Kiis the value in each line of the table and Kaverage is the average of all computed values. The problem with is that we can minimize this function simply by making very small, but not necessarily constant. What we really want is for all the to be clustered around the mean value. A good way to do this is to minimize the relative standard deviationof the K, which is (standard deviation)/average. In cell E5we compute the average value of Kand in cell E6the standard deviation. Cell E7contains the relative standard deviation. Use Solver to minimize cell E7by varying cell A7. Compare your answer with that of Problem 19-13.
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.
Iodine reacts with mesitylene to form a complex with an absorption maximum at 332 nm in CCl4 solution:
(b)Spectrophotometric data for this reaction are shown in the table.Becausewe can say that [mesitylene][mesitylene]tot. Prepare a graph ofversusand find the equilibrium constant and molar absorptivity of the complex.
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