Log in Mr. Cavin 12 years agoPosted 12 years ago. Direct link to Mr. Cavin's post “At 1:15 he said 'spectrom...” At • (19 votes) anderson.o.chen 12 years agoPosted 12 years ago. Direct link to anderson.o.chen's post “A spectrometer is 'An app...” A spectrometer is 'An apparatus used for recording and measuring spectra, esp. as a method of analysis.'. However, a spectrophotometer is ;An apparatus for measuring the intensity of light in a part of the spectrum, esp. as transmitted or emitted by particular substances.;, (48 votes) sethduban 11 years agoPosted 11 years ago. Direct link to sethduban's post “What is the purpose of kn...” What is the purpose of knowing that the solution was measured at 540nm? and was it just coincidence that epsilon = 5.40? • (17 votes) Leigh 11 years agoPosted 11 years ago. Direct link to Leigh's post “It is a coincidence, the ...” It is a coincidence, the question is giving you extra information that is not required to find the answer. (14 votes) ben 11 years agoPosted 11 years ago. Direct link to ben's post “Is mole spelled mole or m...” Is mole spelled mole or mol? Sal spells it both ways. • (6 votes) Ernest Zinck 11 years agoPosted 11 years ago. Direct link to Ernest Zinck's post “*mole* is the _word_ used...” mole is the word used to describe Avogadro's number of particles. mol is the SI symbol for the unit. It is not an abbreviation. (25 votes) Jared Desai 11 years agoPosted 11 years ago. Direct link to Jared Desai's post “I just realized something...” I just realized something. At • (5 votes) FTB 11 years agoPosted 11 years ago. Direct link to FTB's post “Yes, Sal should only keep...” Yes, Sal should only keep 2 significant figures if the length of the vial is to two significant figures. You are correct in your understanding of this. (12 votes) Jannie Khang 13 years agoPosted 13 years ago. Direct link to Jannie Khang's post “what if the length was no...” what if the length was not given? how do i find the molar concentration? • (7 votes) ScienceMon 12 years agoPosted 12 years ago. Direct link to ScienceMon's post “As long as the length is ...” As long as the length is constant, there will be a linear relationship between concentration and absorbance. (8 votes) Michael 12 years agoPosted 12 years ago. Direct link to Michael's post “How did Sal get liter per...” How did Sal get liter per cm times mole? also how can you have a liter per mole? And why did Sal do mole per liter at the end instead of liter per mole? I'm really confused. • (6 votes) Paolo Miguel Bartolo 11 years agoPosted 11 years ago. Direct link to Paolo Miguel Bartolo's post “You're probably referring...” You're probably referring to the unit of the epsilon constant. Usually, constants have complicated units in order to make sure that the answer has the correct unit and that the other units are cancelled out. So according to the Beer-Lambert law, absorbance equals epsilon times length of container (or the length that the light has to travel through to pass through the solution) times concentration. The unit of concentration is molarity, which is moles over liter. The unit of length can be centimeters. Absorbance has no unit. If you would try to multiply the units of epsilon, length, and concentration, you should get the unit for absorbance which has no unit. Multiplying liter per cm times mole, cm, and mole per liter would result to all units cancelling out, resulting in no unit. Sal wanted to find concentration, so he used moles per liter. (6 votes) Oliver Worley 11 years agoPosted 11 years ago. Direct link to Oliver Worley's post “How do you measure the ab...” How do you measure the absorbency of a solution without knowing the concentration? • (5 votes) Paolo Miguel Bartolo 11 years agoPosted 11 years ago. Direct link to Paolo Miguel Bartolo's post “You just need to know the...” You just need to know the intensities of the light before and after it passes through the solution. You first need to find the transmittance of the solution, which is the intensity of the light after it passes through the solution divided by the intensity of the incident light, which is the light before it enters the solution. Then you just get the negative log of the transmittance (use base 10) in order to get the absorbency. Unless the solution has zero transmittance, then the absorbency should be a positive number. I'm not sure what will be the absorbency of a solution with zero transmittance (probably infinite?) (5 votes) Nandagopal M 9 years agoPosted 9 years ago. Direct link to Nandagopal M's post “Will the absorbance be ze...” Will the absorbance be zero when Molarity is zero? Does pure water absorb some light? • (5 votes) Just Keith 9 years agoPosted 9 years ago. Direct link to Just Keith's post “Yes, water will absorb an...” Yes, water will absorb and scatter some light. So, what we do with a spectrophotometer is use what is called a "blank". The blank will NOT contain the substances whose absorbance we're interested in (most of the time the blank is water plus the indicator). And we just treat the absorbance of this blank as if it were 0. This is known as "zeroing out" or sometimes as "blanking out" the spectrophotometer. The way that you do this depends on how sophisticated the method you're using is. So, the answer to the question is that we have to declare something to be 0 absorbance. So, normally we would treat a cuvette containing 0 M of the substance we're interested in as having 0 absorbance -- we just tell the spectrophotometer to treat that as 0. But, strictly speaking, no, it is not actually 0 absorbance. (4 votes) Jim Knight 10 years agoPosted 10 years ago. Direct link to Jim Knight's post “At 4:48, Sal explains tha...” At James Knight • (3 votes) Just Keith 10 years agoPosted 10 years ago. Direct link to Just Keith's post “Beer-Lambert is only appr...” Beer-Lambert is only approximately true. I wouldn't trust it for any absorbance greater than 0.400 myself. (My research required much better accuracy and precision than I student would need, so you might get away with a little higher.) Depending on many circ*mstances, the curve of absorbance vs. concentration begins to flatten out above 0.3 absorbance. The way that we accommodate for this is to analyze standards of known concentrations, and only use those that are within what is called "the linear range" -- in other words where Beer-Lambert applies. (4 votes) dmkgigi 9 years agoPosted 9 years ago. Direct link to dmkgigi's post “So, each time the absorba...” So, each time the absorbance in the graph will increase by depending of the concentration of the solution? • (3 votes) Matt B 8 years agoPosted 8 years ago. Direct link to Matt B's post “Always, because the great...” Always, because the greater the concentration, the more "stuff" lights needs to pass through and therefore less light will pass. (3 votes)Want to join the conversation?
1:15
Hope that helped!
In SI, you use words with words but symbols with numbers. Thus, you would write "two moles" but "2 mol". Note that symbols are not pluralized : it is incorrect to write "2 mols". 7:30 12:15
The biggest difference is that without knowing length, you could not solve for epsilon (the E symbol). Therefore, all unknowns would have to have the same length as your original sample. 4:48
