Shelby and Reagan have been hired by a forensic science lab to analyze trace met

Question

Shelby and Reagan have been hired by a forensic science lab to analyze trace metal content in crime scene evidence. Their current project involves determining zinc content in human hair. Shelby and Reagan prepare keratin protein. Shelby and Reagan need 100 mL of an aqueous 0.3 M HNO_3 solution for this procedure. They remember their chemistry training and plan to "add acid to water", and add the necessary volume of concentrated (16.0 M) HNO_3 to a sufficient quantity of water. Reagan decides to use a 100-mL volumetric flask to prepare the solution. The flask is filed about three-quarters full of deionized water and the acid is measured using a 10-mL graduated cylinder. After mixing the solution well, Reagan fills the flask with enough deionized water to bring the level to the mark on the neck and mixes the solution well again. Shelby, on the other hand, uses a 100-mL graduated cylinder to measure 100 mL of deionized water, pours it into a beaker, adds the HNO_3 from a 10-mL graduated cylinder and stirs well. a. How many milliliters of the concentrated HNO_3 are needed to make 100 mL of 0.3 M solution? b. What advantages are there to using a volumetric task to prepare the HNO_3 solution? Explain. c. What advantages are there to using the graduated cylinder to prepare the HNO, solution? Explain. d. Consider the photograph of a 10-ml graduated cylinder at the right. Estimate the uncertainty in the volume of concentrated HNO, that Shelby and Reagan measure using this glassware. e. As they are working in the lab, Shelby and Reagan notice the different approaches they have taken to making this solution. What are the advantages and disadvantages of each approach? Which student made the better choice? Why? f. Considering the role of the nitric acid solution in the hair analysis, how important is the accuracy of the concentration? g. If another student asked you for advice on what glassware to use in preparing the HNO_3 solution, what would you tell him or her?

Explanation / Answer

28. HNO3 solution

a. Volume of concentrated HNO3 required to prepare dilute solution

= 0.3 M x 100 ml/16 M

= 1.9 ml

b. using volumetric flask the solution can be easily prepare by first adding 1.9 ml of concentrated HNO3 and then adding rest water to the flask upto the mark without error.

c. In a graduated cuyllinder, the volume of water and HNO3 both are measured separately and mixed together.

d. the error associated with the measurement would be +/-0.4 ml

e. Student using volumetric flask made a better choice as the error associated with volumetric flask is minimal.

f. The HNO3 solution must be very accurate when using it for the given reaction.

g. For preparing the HNO3 solution, the best glassware would be volumetric flask and graduated pipette to transfer HNO3 to the volumetric flask.

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