1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Beer's Law

  1. Jul 4, 2010 #1
    1. The problem statement, all variables and given/known data

    I've run into a problem and fear I'm just approaching from the wrong angle.

    From Ebbing Experiments in General Chemistry 9e 14B if you have it to reference..

    It's the classic Fe + SCN > Fe(SCN)

    In our experiment we are using SCN as the limiting reactant and overwhelming amts of Fe so that the initial [SCN] should equal the final [Fe(SCN)] at equalibrium. We are using a spectophotometer to measure absorbance at 450nm

    2. Relevant equations
    Our Keq is obtained by: [Fe(SCN)]/[Fe] [SCN] resulting in m=∑xy/∑x^2 ; K = 2.704 10^4

    Our fist 5 series we varied [SCN] keeping [Fe] constant.

    6-10 we varied [SCN] and keeping [Fe] to a different constant.

    11-15 we varied [SCN] and keeping [Fe] to yet a different constant.

    3. The attempt at a solution

    So we have 3 constant [Fe].

    Since the Keq = [Fe(SCN)]/[Fe] [SCN] and [Fe(SCN)] =[ SCN] the results should be 1/[Fe]...right?

    [Fe] in 5-10 = 3.57E-4 ..... inv= 2.8E-3 so K = 2.8E-3
    [Fe] in 5-10 = 7.14E-4 ..... inv= 1.4E-3 so K = 1.4E-3

    Using Beer's Law A=kc I have from for example #5 A=.216 (@450nm) K = 2.8E-3
    Solving for c [Fe(SCN)]? gives me 605 moles!

    Subsequent equations yield equally baffling results (to me baffling!)

    Via plotting data from the first 5 experiments in Excel and crunching number in a linear regression analysis formula:

    Since m=∑xy/∑x^2; K = 2.704 10^4 so my k values are reasonable but the 605 moles is not! ...(is it?)

    What am I not seeing here?

    Perhaps I'm not grasping what the "c" in A=kc is for as I already know the Keq for [FE(SCN)] and in fact ALL my concentrations.....and A's and K's.........

    Last edited: Jul 4, 2010
  2. jcsd
  3. Jul 6, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No. Given the initial concentration of thiocyanate is equal to [SCNi], let x equal to the amount of [SCN] converted to [Fe(SCN)]. The concentration of thiocyanate in the equilibrium expression is therefore [SCNi-x] and the concentration of [Fe(SCN)] you measure is simply 'x'. Substitute these values into your equilibrium equation and solve for x.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook