Chem Lab C

[binary010101]

Can someone explain Beer's Law?

[scienceolympiadist]

"relates the absorption of light to the properties of the material through which the light is traveling"
http://en.wikipedia.org/wiki/Beer%27s_law

looks confusing. I thought it was just a linear relationship between absorbency and concentration?

[peoneill]

Beer's Law is typically expressed as A=(e)bc only the (e) is actually the greek letter epsilon I can't type (either because there's no function for it or I'm just not smooth enough).

A = absorbance, which is a function related to the transmittance. Transmittance is how much light of a certain wavelength actually passes through the solution. Absorbance is a transformation of transmittance - the spectrophotometer does the math for you (even professionals don't usually know the details, spectrophotometers always present their results to you in terms of absorbance). Absorbance is more useful than transmittance because it has a LINEAR relationship to concentration, as you've noted.

(e) = molar absorbtivity coefficient (or constant), which is a constant that is different for each solution. This constant is the slope of the line in a typical Beer's Law plot.

b = the thickness of the path the light has to take passing through the solution... because most spectrophotometers always use a standard sized test tube (say the width is always 1cm for all measurements) this is essentially NOT a factor and can simply be omitted from the equation. Sometimes people call b "l" for length, but those people are lonely and should adopt a pet. Something affectionate, like a cat or a dog, not like a fish or a lizard.

c = the concentration of the solution; note that any units can be used so long as they are linearly based (i.e. you can't use pH or a logarithmic measure of concentration, but ppm, %, molarity, g/mL, etc. are all fine) AND so long as they MATCH THE UNITS OF (e). Normally you're just figuring out (e) for yourself so it's whatever units you feel like calculating it in. Typically real world chemists would be working in either ppm (for super dilute stuff, which is often what spectrophotometry is used for) or in M (molarity).

So yes, it's typically just linear, as A=(e)C meaning absorbance is directly proportional to concentration (remember (e) is just a constant, making it the slope of the line relating A to C). Also, Beer's Law graphs pass through the origin. My undergrads ALWAYS screw that up even after I warn them that they're going to. Bozos.

[Avia]

I get what Beer's Law IS, but what kind of problems generally require the use of Beer's Law? Are they straight forward "Using Beer's Law, solve for this variable" or are they word problems or situation problems that require you to know intuitively that you need to use Beer's Law?

[zeebz -_-]

for Beer's Law,my team hd to determine what law it was and stuff like that. but,we didn't have a lab for that competetion and so all we had was the test.

[peoneill]

In 2002 at nationals (the 2010 rules are a copy paste of the 2002 rules) we were given a spectrophotometer and one solution with a known concentration as well as one with an unknown concentration.

First we mixed up reference solutions of multiple concentrations using the known solution. Then we spectrophotometerized 'em and made a standard "curve" (which is not so much a curve as it is a completely straight line...) by plotting absorbancies with respect to concentration. Then we spectrophotometerized the unknown solution and figured out it's concentration from our standard curve. Then they asked us to determine the value and units of the molar absorbtivity constant.

[tmanneo]

Wow, Chem Lab doesn't look too bad. I love getting practice for these labs through my AP Chem class. The lab before the last lab I did was Beer's Law, and I used all the absorbencies and such. The annoying thing was making the dilution though. I had no clue what ratio to use first.

What exactly do they mean by the first lab? "Use the concept of density to experimentally determine the concentration of a solution?"

[peoneill]

Concentration and density are not independent functions: therefore one can be used to determine the other. As density is easy to directly measure and concentration is difficult to directly measure, that's a useful relationship.

I assure you this is one of the more difficult sets of rules for chemistry lab. There's a lot of room for wicked hard questions in both topics, much MUCH more difficult than dilution ratios and density labs - make sure you're fully digesting those rules. An AP level textbook will present excellent prep if you do the kinetics and solution chemistry chapters thoroughly (or whatever they call the chapter with colligative properties).

[winneratlife]

My team just lost their member for this event, so I'm considering taking it. Problem: Invitational is Saturday.
1. Do you think I can do this, or should I just let someone else? (I have a really minor chem background, but it seems I'm the only one with any work ethic at all on this team (after break, there were like 3 people out of 30 who had even started studying.)
And, 2., if I should take this event, does anybody know a place were I can get a crash course?

EDIT: Haha, got stuck doing it anyways...

[Flavorflav]

What are some of the labs people are running into at invitationals? Anything unexpected?