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What is the concentration of chromate?

  1. Oct 16, 2012 #1
    What is the concentration of chromate??

    1. The problem statement, all variables and given/known data

    A solution of K2CrO4 has an absorbance value 0.555 in a cell of 1.00cm path-length at a wavelength of 370nm. The value of the molar absorbtivity is 5.25x103M-1cm-1 for K2CrO4 at the wavelength. What is the concentration of this chromate solution? Report with Three significant figures.



    2. Relevant equations

    Molar mass of K2CrO4 - 194.2
    absorbance value is .555 at a wavelength of 370nm
    molar absorbtivity - 5.25*10^3M^-1cm^-1

    3. The attempt at a solution

    I would attempted this but I have no clue how to get it to work really hard lol, I dont understand how I can even go about starting this please please help!!!
     
  2. jcsd
  3. Oct 16, 2012 #2

    Borek

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    Staff: Mentor

    Re: What is the concentration of chromate??

    What is the law that should be used? The one that combines concentration and absorbance?
     
  4. Oct 16, 2012 #3
    Re: What is the concentration of chromate??

    Beer's law right??
     
  5. Oct 18, 2012 #4
    Re: What is the concentration of chromate??

    So I have tried this problem so many times now lol. I have one more attempt at the problem for credit. So I first tried to do:

    absorbance = eLc

    0.555 = (5.25 * 1.00) *C which I got C = 5.25 *1.00/.555

    But this was wrong so I attempted to do:

    C = A/(eb)

    A = absorbance
    e = mole absorabtion
    b = path length
    c = concentration

    0.555/(5.25)(1.00)

    But this was wrong as well I really need help!!! This problem has given me so much trouble and Ive looked though my chem book still cant find an example of it nor online......
     
  6. Oct 18, 2012 #5

    Borek

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    Staff: Mentor

    Re: What is the concentration of chromate??

    You need to pay attention to units and to numbers. According to your first post question says that molar absorbtivity is 5.25x103M-1cm-1 - which I guess means 5.25x103M-1cm-1. Yet you calculate concentration using value of 5.25 - no wonder your result is thousand times off.
     
  7. Oct 18, 2012 #6
    Re: What is the concentration of chromate??

    Wow lol got it!!!! Thanks man!!

    So it should be -

    C = A/(eb)

    A = absorbance
    e = mole absorabtion
    b = path length
    c = concentration

    0.555/(5250)(1.00) which is -
    1.06E-4 which is 0.0000106

    Lets say I got it right, haha and a 100 on the prelab thanks for the common sense help lol

    :smile:

    minor things right lol im pursuing a Computer Science degree chem is killing me lol just the little stuff too
     
  8. Oct 19, 2012 #7

    Borek

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    Staff: Mentor

    Re: What is the concentration of chromate??

    1.06e-4 means 1.06*10-4 - note the correct use of subscripts. "E-4" notation was specifically invented in the computer prehistory to allow printing the exponential part with a standard ASCII characters.
     
  9. Oct 19, 2012 #8
    Re: What is the concentration of chromate??

    I'm trying hard not to make trouble for myself here, but... You do realize that everyone's TI calculators (and every Casio or other brand scientific and graphing calculators that I've seen recently) uses the EE function for scientific notation, right? You used a small 'e', but I notice that chaz didn't in the post that you're quoting in order to berate him about semantics.

    Don't get me wrong, I very much appreciate your participation here (I'm sure that my professor does as well, for that matter :wink:). I've been in similar shoes as you are wearing here myself though, and I know how easy it is to get caught up with this sort of thing.

    I've been looking around here a bit for a little while now, and my impression is that those of you who are answering have gotten a bit too unwilling to provide answers. We all have tons of problems available, so I don't see how answering one or two questions with solutions is harmful. If the same person floods the board with a multitude of problems then that's obviously an issue, but... one good solution can be tremendously helpful to someone who is actually doing the work, but will be essentially useless to someone who is trying to get by with the bare minimum effort.

    I'm off on a bit of a rant here, so my apologies for that. It seems as though Chaz has had his question answered already though, so I don't think that I'm hurting anything. I'm trying to participate here as well, rather than just trying to leach solutions and answers from others. I'm really uncertain about how much I should say though, given how little the "admins" and/or "moderators" are themselves saying.
     
  10. Oct 20, 2012 #9

    Borek

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    Staff: Mentor

    Re: What is the concentration of chromate??

    1e4 and 1E4 means the same, you will see it quite often on the printouts as its the standard way of formatting program output - see for example http://en.wikipedia.org/wiki/Printf_format_string#Format_placeholders. I was mainly referring to the unnecessary superscript. It shows now and then, I guess people are combining 2.3*10-4 and 2.3e-4 (or 2.3E-4) producing 2.3E-4 - which doesn't mean anything.

    Problem is - if students are shown how to solve the problem, they know how to solve this particular problem and they are hopelessly lost when the problem they are facing is different. However, when they are trained to solve problems on their own from the first principles, they are much more flexible and they are not thrown off just because question differ by some details. Note that even our template is designed to force this kind of thinking. That's all for a purpose. We know from experience (and not experience collected just here, at PF) that students that solve problems on their own know more than those that aped the solution.

    But this is quite off topic now.
     
  11. Oct 20, 2012 #10
    Re: What is the concentration of chromate??

    Ah:
    My bad, then. That wasn't clear from what you were saying, but... no big deal.

    Anyway, you're describing the role of this forum to be a replacement for the classroom rather than a sort of adjunct tutoring center. If that's the common attitude around here, then I'm clearly in the wrong place. I don't want to have any part in a web forum that is seeking to replace the classroom.
     
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