CH228 Virtual TA - Frequently Asked Questions (FAQs)
General Chemistry Laboratory
Dr. Deborah Exton

Spectroscopy Lab

Question: I am confused. The value of the Rydberg constant which I determined from the slope of my graph is close to the literature value, but the sign is negative instead of positive. What did I do wrong?
Answer: You didn't do anything wrong. The problem is that the values of E which you calculated from the wavelength were positive (right?), but we know that since the emission of light is an exothermic process, the sign for the energy is actually negative. If you had plotted negative values for the energy, your slope (and Rydberg constant) would be positive. You don't need to re-draw the graph, just recognize that the sign for the slope is wrong.

Question: I only see three mercury lines. Am I seeing the blue Hg line or the violet Hg line?
Answer: The blue line is more intense than the violet line. If you see only one line at the blue end of the spectrum, it is almost certainly the 435.8 nm blue line.

Question: Where should I look to find the violet mercury line?
Answer: The distance between the blue and violet mercury lines is just about the same as the distance between the yellow and green mercury lines (see illustration). Look at the separation between the yellow and green mercury lines, and then look that far to the right of the blue mercury line -- that's where you should find the violet line.

Question: I think I'm looking in the correct place for the violet mercury line, but I still can't see it. What should I do?
Answer: The violet mercury line is rather faint. For the purpose of finding this line, you may want to get your spectroscope very close to the mercury lamp. Try to make sure that your entrance slit is parallel with the light emitting part of the Hg lamp. This will ensure that as much light as possible gets into your spectroscope. If that doesn't work, try widening your entrance slit a bit.

Question: I only see three hydrogen lines. Am I seeing the blue H line or the violet H line?
Answer: The blue line is more intense than the violet line. If you see only one line at the blue end of the spectrum, it is almost certainly the 434 nm blue line. The violet hydrogen line is very faint. Typically, only a very few students are able to locate it.

Question: Okay, I only see three hydrogen lines, and you say I'm seeing the blue line but not the violet line. But the line I see sure looks violet to me. What's up with that?
Answer: The blue line for hydrogen (and for mercury) falls at the violet end of the range of colors we would call blue. It does look kind of violet when viewed in isolation. However, if you see the blue and the violet lines next to one another, there is no question about which is "blue" and which is "violet". Your TA has a sheet of Crescent board that is very close in color to the blue of the hydrogen line. Also, we've tried to reproduce the colors of the blue and violet lines here. What color you see on your monitor depends a lot on the monitor that you are using, so the graphic below should just be considered an approximation of the actual colors. (Macintosh computers seem to render the colors most accurately; some PCs make the blue too blue.)

Question: I saw a yellow hydrogen line. Why isn't it listed on our handout?
Answer: The emission lamps we use in this lab wear out relatively quickly. Sometimes you can see a yellow line when looking at a hydrogen lamp that is nearly worn out. The yellow line is not coming from hydrogen. What do you think is causing the yellow line? What is the yellow line's wavelength? Do any of the lines in the metal spectra you viewed as part of this lab have very similar wavelengths?

Question: The lower energy level is the same for all of the hydrogen lines we saw in this lab, right?
Answer: Correct! You will be able to determine what that value is when you make your graph.

Question: I'm color blind. What should I do for this lab?
Answer: Let your TA know. If you describe the positions of the lines you see, your TA can tell you what color they are (assuming that he isn't color blind, too!).