Question

Name: Determination of Solution Concentration by Spectrophotometry PRELAB QUESTIONS 1. A student measures the absorbance of an unknown after placing a sample in a cuvette which contained some water. How will this affect the concentration he reports? 2. Explain why it is all right to leave water in the volumetric flask before you put in the 10.00 mL of your unknown. 3. A 1.3 x 10-M solution of an unknown substance has an absorbance of 0.345 at its max. a) Calculate the molar absorptivity, E, of the substance at this wavelength, using a sample cell with a 1.00 cm path length. b) Calculate the absorbance of an aqueous solution of this substance at its Amax ifits concentration is 2.5 x 10-6 M.

Answer 1

1) As we know that:  $A=\varepsilon cl$

where, A= absorbance, $\varepsilon$ = molar absorptivity, c= concentration and l= path length.

From the above mentioned equation it is clear that the absorbnce (A) of a solution is directly proportional to concentration (c). Thus, if the cuvette contains water before placing sample then the concentration of solution will decrease therby decreasing the absorbance of the solution.

2) It is not compulsory to leave water in the volumetric flask before adding it in the 10 ml of unknown solution. But it is alright if a person do se because the idea behind it, is to minimize the chances of error and we wil be able to add an accurate amount of solution.

3) and 4)

Given 1-3X10 A-O.345 ee 1 cm Calculate E Cmolar absonptivily) Now know that we A= Ecl A G = cl 0.345 26538 .4615 1 (1.3 X165 M J4 cm) 2653 8-4615 L/molfem E- 26538.461S Mcm When substance Of lnis Calculale the absotbcnce M. 2.5 Xlo A = As 5 A =26S3 B.4615mot cyx(a u)xbsx10 Theretore 0.6635 A - concent saton is Jthe cample M) i A beosbcurce B 0.6635 changed to (3-5710s

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