Chapter 18: Q7 P (page 456)
18-7: How do transmittance, absorbance, and molar absorptivity differ? Which one is proportional to concentration?
Absorbance is proportional to concentration, Molar absorptivity is the constant of proportionality between absorbance and concentration.
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Preparing standards for a calibration curve.
(a) How much ferrous ethylene diammonium sulfateshould be dissolved in a 500mL volumetric flask withto obtain a stock solution with?
(b) When making stock solution (a), you actually weighed out 1.627 g of reagent. What is the Fe concentration in role="math" localid="1668357579931" ?
(c) How would you prepare 500 mL of standard containing 1,2,3,4, andinfrom stock solution (b) using any Class A pipets from Table 2-4 with only 500 -mL volumetric flasks?
(d) To reduce the generation of chemical waste, describe how you could prepare 50 mL of standard containing, andinfrom stock solution (b) by serial dilution using any Class A pipets from Table 2-3 with only 50mL volumetric flasks?
The absorbance of a solution of a compound is 0.822 at a wavelength of 266nm in a 1.00-cm cell. Calculate the molar absorptivity at 266nm.
Semi-xylenol orange is a yellow compound at pH 5.9but turns red when it reacts with . A 2.025 - mLsample of semixylenol orange at pH 5.9was titrated withwith the following results:
Total role="math" localid="1663647483742" added | Absorbance at 490nmwavelength | Total added | Absorbance at490nmwavelength |
0.0 6.0 12.0 18.0 24.0 30.0 36.0 | 0.227 0.256 0.286 0.316 0.345 0.370 0.399 | 42.0 48.0 54.0 60.0 70.0 80.0 | 0.425 0.445 0.448 0.449 0.450 0.447 |
Make a graph of absorbance versus microliters of added. Be sure to correct the absorbances for dilution. Corrected absorbance is what would be observed if the volume were not changed from its initial value of 2.025Ml. Assuming that the reaction of semi orange with has a 1 : 1stoichiometry, find the molarity of semi-xylenol orange in the original solution.
Nitrite ion, is a preservative for bacon and other foods, but it is potentially carcinogenic. A spectrophotometric determination ofmakes use of the following reactions
Here is an abbreviated procedure for the
determination:
1. To 50.0ml of unknown solution containing nitrite is added 1.00mL of sulfanilic acid solution.
2. After 10min , 2.00mL of 1 –a minonaphthalene solution and 1.00 mL of buffer are added.
3. After 15 min, the absorbance is read at 520 nm in a 5.00-cm cell.
The following solutions were analyzed:
A. 50.0 mL of food extract known to contain no nitrite (that is, a negligible amount); final absorbance =0.153.
B. 50.0mL of food extract suspected of containing nitrite; final absorbance \(=0.622\).
C. Same as B, but with added to the 50.0-mL sample; final absorbance =0.967.
(a) Calculate the molar absorptivity, of the colored product. Remember that a \(5.00\)-cm cell was used.
(b) How many micrograms ofwere present in 50.0mL of food extract?
Vapor at a pressure of bar from the solid compound pyrazine had a transmittance of 24.4% at a wavelength of 266nm in
a 3.00 - cm cell at 298K
Pyrazine
(a) Convert transmittance to absorbance.
(b) Convert pressure to concentration (mol/L) with the ideal gas law (Problem 1-18).
(c) Find the molar absorptivity of gaseous pyrazine at 266nm.
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