POST-LABORATORY QUESTIONS 1. You have at your disposal three pipets a 10-ml volu
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POST-LABORATORY QUESTIONS 1. You have at your disposal three pipets a 10-ml volumetric pipet, a 20-mL serological pipet, and a 1o- ml. serological pipet. Which pipet would you use to dispense 17 mL of a liquid? Explain your choice. 2. You measured the absorbance of a solution of NADH in a cuvette 1 cm in path length and found it to cm1, what is the concentration of NADH be 0.75. If the extinction coefficient of NADH is 6220 M in this solution? Show your work 3. Which method of determining your unknown would be more reliable: reading a value from your graph or calculating using a computer program? Explain your answer.Explanation / Answer
1) A volumetric pipette has no graduations and the 10 mL volumetric pipette can only dispense 10 mL of the test solution. No intermediate volume can be dispensed by the 10 mL pipette.
A serological pipette is a graudated pipette. 10 mL and 20 mL serological pipettes have 0.1 mL graudations, i.e, serological pipettes can dispense 0.1 mL volume of the test solution. Since we wish to dispense 17 mL test solution, hence the 20 mL serological pipette is the best choice and the volume dispensed will have a tolerance of 0.1 mL.
2) Use Beer’s law:
A = *C*l
where A = 0.75 is the absorbance of the NADH solution; = 6220 M-1cm-1 is the molar extinction coefficient of NADH and l = 1.00 cm is the path length of the solution. C is the concentration of NADH. Plug in values and obtain
0.75 = (6220 M-1cm-1)*C*(1.00 cm)
=====> C = (0.75)/(6220 M-1) = 1.2058*10-4 M
The concentration of NADH is 1.2058*10-4 M (ans).
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