(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Hi,

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.........

Thanks

Whalstib

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Beer's Law

**Physics Forums | Science Articles, Homework Help, Discussion**