Question

F17 UTA-401 4. A test for drug use involves ad residue is present in the sample the solution Because other things in urine may give sig concentration of 5.0x10 M drug residue in drug use involves addition of 10.00 mL of colorless reagent to 10.00 mL urine. If drug it in the sample the solution turns blue (A max = 605 nm, ε = 18,200 M'cm). ngs in urine may give slightly positive results, only tests that measure a minimum of 3.0x10M drug residue in the original urine sample are considered positive for hen you measure the sample using the test described above and get an Abs of 0.539 b= l cm). Do you conclude that this person has been using illegal drugs? Explain. 5. A Student decides they want to analyze the green Fruit Loops that we threw away. They obtain the spectrum pictured below. Using what you learned about the food dyes in this experiment, propose how you think the green color was created in these loops. 0.3 Abs 02 Wavelength [m] 103

Answer 1

4. We need to determine the concentration of "drug" (quotes because we don't know if it will be above the determine threshold value of 5.0x10^-5 M) in the sample. We can do this by using the Beer-Lambert law, which states that:

$A=\varepsilon \cdot l\cdot c$

Where A is the absorbance, epsilon is the molar absorptivity, l is the optical path length (which is called "b" in this excercise) and c is the concentration. We can rearrange for c and calculate:

$c=\frac{A}{\varepsilon \cdot l}=\frac{0.539}{18200\frac{1}{M\cdot cm}\cdot 1cm}=2.96x10^{-5}M$

This is the concentration of the supposed drug in the measured sample, which is a mixture of 10.0 mL of urine and 10.0 mL of color reagent. This means that the concentration in the urine is double the one obtained by the measurement, because it has been diluted to half its original concentration (because the volume has been doubled from 10 mL to 20 mL). So, the concentration in the urine sample is:

$c=2\cdot 2.96x10^{-5}M=5.92x10^{^{-5}}M$

Which is above the given threshold, so the person has consumed illegal drugs.

5. When compounds absorb visible light, the color we see is the color corresponding to those wavelengths which HAVE NOT been absorbed. The visible electromagnetic spectrum is:

400nm 450nm 500nm 550nm 600nm 650nm 700nm

As can be seen by the given spectrum, the range between 500 and 550 nm is the least absorbed. This means that the light in this range will be reflected by the object and the human eye will perceive the corresponding color. As can be seen in the image, the color corresponding to the non-absorbed range is green; this is why the fruit loops have that color.