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13 Dec 2019

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PNP (2.8 mg) (MW = 140) was dissolved in 20 mL of 0.1 M NaOH. A small aliquot of this solution (0.1 mL) was diluted with 4.9 mL of pH 8 buffer (0.1 M), and the absorbance at 415 nm was determined. If the molar absorptivity is 15,000 L/cm/mol at this wavelength and pH, and the path length is 1 cm, what absorbance reading do you expect?

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17 Dec 2019

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A 50 mL stock solution of a food colour having a molecular weight (molar mass) of 316.15 g mol-1 was made. 1 mL of this stock solution was then diluted to 50 mL. The absorbance of the diluted solution at 450 nm wavelength was 0.53 using a 1 cm cuvette. It is known that this compound has a molar absorptivity (extinction coefficient) of 389 mol-1 L cm-1 at this wavelength. What was the mass of the food colour dissolved in the stock solution in mg?

Absorbance = Extinction coefficient Ã Concentation Ã Path length of cuvette ( A = ? c d )

i'm having a lot of difficulties with this exercise, pleasesolve and explain. i appreciate your help and time.

The acid/base indicator HIn undergoes the following reaction indilute aqueous solution: HIn (color 1) < === > H+ + In-(color 2)

The following absorbance data were obtained for a 5.00 x 10-4 Msolution of HIn in 0.1 M NaOH and 0.1 HCl. Measurements were madeat a wavelength of 485 nm and 625 nm with 1.00 cm cell. 0.1 M NaOHA(485 nm)=0.052, A(625 nm)=0.823 0.1 M HCl A(485 nm)=0.454, A(625nm)=0.176

In the NaOH solution, essentially all of the indicator ispresent as In-; in the acidic solution, it is essentially all inthe form of HIn.

a) Calculate mole absorptivities for In- and HIn at 485 and 625nm.

b) Calculate the acid dissociation constant for the indicator ifa pH 5.00 buffer containing a small amount of the indicatorexhibits an absorbance of 0.472 at 485 nm and 0.351 at 625 nm (1.00cm cells)

c) What is the pH of a solution containing a small amount of theindicator that exhibits an absorbance of 0.530 at 485 nm and 0.216at 625 nm (1.00 cm cells).

d) A 25.00 mL aliquot of a solution of purified weak organicacid HX required exactly 24.20 mL of a standard solution of astrong base to reach a phenolphthalein end point. When exactly12.10 mL of the base were added to a second 25.00 mL aliquot of theacid, which contained a small amount of the indicator underconsideration, the absorbance was found to be 0.306 at 485 nm and0.555 at 625 nm (1.00 cm cells). Calculate the pH of the solutionand Ka for the weak acid.

e) What would be the absorbance of a solution at 485 and 625 nm(1.25 cm cells) that was 2.00 x 10-4 M in the indicator and wasbuffered to a pH of 6.000?

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