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Beer's Law 6,780 views

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Beer s Law states that a solution's absorbance of light at a certain wavelength is directly proportional to the concentration of that solution. Beer s Law is mathematically expressed as M1/A1=M2/A2.

Alright so in your unit of solutions you might come across Beer's Law and Beer's Law states the absorbance of light in a certain wave length is directly proportional to the concentration of a solution. What that essentially is saying is the darker the solution the more concentrated it is. So if you were to look over actually at these 2 solutions, these are both tea solutions you'd automatically know that this guy is more concentrated than this guy and that's because this guy is darker. It's actually absorbing more light than this guy, is absorbing less light. That's basically what Beer's Law is stating and you can actually, there's an instrument out there called the spectrometer that actually talks about this measure of this, the absorbent, how much absorbent solution actually has. So if we actually we go over back to Beer's Law, Beer's Law states that the molarity in a particular solution divided by its absorbency is equal to the molarity of the second solution divided by its absorbency. So how do you get that, the absorbency of light? So you go over to a spectrometer and what that does is, you have to make it out a certain wave length. So you're going to actually I haven't measured give off energy or light at certain wave length and that's what's going on here, it's going to actually filter on some light and so to go through your sample and actually going to read how much light it actually absorbed.

And the more concentrated the solution is, the more absorption is going to have. So let's actually go over here and do a problem relating all these together. So let's say we have a solution with a concentration of 0.14 molar, it's measured to have an absorption of 0.43. What that 0.43 is actually indicating is that it'll absorb 43% of the light at that particular wave length. Another solution of the same chemical is measured under the same conditions, it has absorbance of 0.37, 37% what is the concentration? Well let's think before we even get to this problem let's think what it should be. Okay so the absorbency is lowering so the concentration should also be lowering. So it's actually going to be more dilute. So we're going to say that the first molarity which is 0.14 molar divided by it's absorbency which is 0.43 is going to equal the absorbency of the new concentration or the diluted concentration that we know of in this case, is equal to, so divided by 0.37 and if you cross multiply 0.37 times 0.14 divided that 0.43 you're going to end up with 0.12 molar solution is your new solution. Which is exactly how we predicted, the absorbance is going down, the concentration is also going down. This is actually a good example of Beer's Law in action.