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### Course: AP®︎/College Chemistry>Unit 3

Lesson 13: Beer–Lambert law

# Spectrophotometry and the Beer–Lambert Law

Spectrophotometry is a technique that uses light absorption to measure the concentration of an analyte in solution. The amount of light absorbed by a solution is related to the analyte concentration by the Beer–Lambert law, which is expressed as follows: A = εbc, where ε is the molar absorptivity of the analyte, b is the path length (the distance the light travels through the solution), and c is the concentration of the analyte. Created by Sal Khan.

## Want to join the conversation?

• About Absorbance at . When Sal says "The negative log of T" does he mean you take the logT and multiply it by negative 1 (-1), or does it mean 1 over the log of T (1/logT)? Like how 10^-1=1/10.
• He means either (-1)×logT or log(1/T).
Note: log(1/T) = log1 - log T = 0 - log T = -logT.
They both mean the same thing.
• Hi Sal, in my A-level course we have to learn the colors of transition metals and about d-d transition and metal-ligand charge transfer, is there any way to be able to calculate the colors/wavelengths? as its hard to remember the color changes as my mind is quite mathematical
• On an atomic level, are bonds non-transparent too? And can you concentrate a liquid to where no light can get through?
• Electromagnetic bonds are indeed invisible. It's like holding two magnets a couple inches apart; there's nothing holding them together but the electromagnetic force. You probably could concentrate a liquid so no light got through: if the solution is 99.99 solvent and the rest solute, then you essentially have a solid block which no light can get through.
• On the graph at , is there a unit in which absorption is measured?
• According to
Absorbance doesn't have any units because its the ratio of the amount of light that passes through a solution compared to the amount of light that is passed into it. Sometimes you'll see absorbance units (AU) as its units.
Hope this helps :)
• at he uses the word opaque?? what does that mean?? less solutable??
• Hi Zack,

An opaque object does not allow light to pass through it (ie. a deck of cards, a sheet of aluminum foil, etc.). An easy way to check for opacity is to completely cover a lit flashlight with the object in question. If no light passes through, the object is opaque. However, if some light, or all the light, can pass directly through the material, the object is not opaque.

Opacity can be compared to transparency and translucence. A transparent ("see-through") object allows all light to pass through it (ie. a sheet of plastic or clear glass), and a translucent object allows only some of the light to pass through it (ie. a sheet of wax paper).

Here's a good article with some more on opacity, including a quantitative definition: http://en.wikipedia.org/wiki/Opacity_(optics)

Hope that helps :)
• At something was said about shining a wavelength of light is specifically sensitive to the solute. my question is why is the spectrophotemeter set at certain wavelengths, why cant it just be set at any wavelength of our choice?
• That is because chemicals only absorb very specific wavelengths of light. This is why we can use a spectrophotometer to measure the concentration of a specific chemical. If the wavelength of the light is wrong, then the light won't be absorbed.
• thanks for your answer but just one more thing. What i asked abt the frequency ,is it true in the case of wavelength
• Frequency and wavelength are just different ways of expressing the same thing. Frequency = c / wavelength (where c=speed of light).
• What is molarity? Sal didn't mention it any of the previous videos, so I'm surprised at it showing up so suddenly. Can anyone help?