Chemistry 150

Please have the following pages ready before class on Tuesday, January 8. Note that the different parts will be standard divisions in all lab writeups. For this particular writeup, please write an abstract and paper-clip it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up is due in class on Wednesday, January 16.

This lab was taken from the Chemistry 125 class at Cal Poly San Luis Obispo and an on-line version of the procedure can be found at:

http://chemweb.calpoly.edu/chem/125/125LabExp/ComplexIon/ComplexIon.html

And this is what your lab notebook should look like:

Your name, your partner’s name, date of experiment

Lab 1: The spectroscopic determination of the stoichiometry of an iron (II) complex

Part 1. Purpose

Go to the website above and read the introduction (first page). Summarize the experiment’s goal in a sentence.

Part 2. Materials and methods

Equipment: Spec 20 spectrophotometer, sample tubes, 1 and 5 mL micropipetters, micropipetter tips for both and 2 100 mL beakers for using as reservoirs for the stock solutions

Chemicals: Fe (II) ion stock solution (made from ferrous ammonium sulfate), 1,10-phenanthroline stock solution

Sketch the setup as you are using it and label the various pieces of equipment.

Part 3. Procedure

Note that there are three procedure sections; you may print out each of the sections and paste them into your lab book here (it is not necessary to generate a second copy to paste onto the pages you will turning in). Trim off any irrelevant parts (like the “back” arrow) and any sample tables (you will draw your own below).

Part 4. Original data

For the sake of your being able to write (and make mistakes and cross-outs), you should make the squares in these tables larger when you copy them into your lab book.

Table 1: Mixtures of different amounts of iron (II) ion and the ligand

Tube	1	2	3	4	5	6	7	8	9	10	11	12
Fe(II) stock solution	4.50	4.00	3.50	3.00	2.50	2.00	1.50	1.25	1.00	0.75	0.50	0.25
1,10-phenanthroline
Total Volume (mL)	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00	5.00

Table 2: Tube 5 solution absorbance at different wavelengths

Wavelength (nm)	460	470	480	490	500	510	520	530	540	550
Absorbance

Wavelength of maximum absorption (l_max) ______________________ nm

Table 3: Mixtures of different amounts of iron (II) ion and the ligand

Trial 1:

Tube	1	2	3	4	5	6	7	8	9	10	11	12
Absorbance

Trial 2:

Tube	1	2	3	4	5	6	7	8	9	10	11	12
Absorbance

Trial 3:

Tube	1	2	3	4	5	6	7	8	9	10	11	12
Absorbance

Part 5. Calculated results

Generate the graph mentioned in procedure section 3, either on your calculator or on a graphing program (such as Excel). Print the graph out and attach it here. Do not forget to calculate the correlation coefficient (r²) and the equation of the line for each of the two lines requested. Write both sets of information on the graph. Don’t forget to label the graph axes and give the graph a title.

Calculate the mole percent from the intersection of the two lines and convert that to a number of 1,10-phenanthroline ligands (be careful on sig figs here — what was your least precise measurement up to this point?). Finally, determine the chemical formula of the complex; i.e., write Fe (phenanthroline)_x. where x is the whole number of ligands (note that you will have to round your previous result).

To get an estimate of the error, you will need to sketch in four more lines on your graph. For the ascending series, connect the two points that will give the highest possible positive slope; also connect the two points that will give the lowest possible positive slope. For the descending series, connect the two points that will give the highest possible negative slope; also connect the two points that will give the lowest possible negative slope.

Calculate the mole percent of the intersection of the highest possible positive slope line with the lowest possible negative slope line; then calculate the mole percent of the intersection of the lowest possible positive slope line with the highest possible negative slope line

Take each of these mole percents and convert them into a number of 1,10-phenanthroline ligands. Report the error as, for instance, “The number of ligands per iron ion is 3.445 +0.344/-0.233”. Notice that the high/low values may not be symmetric around the calculated value!

Part 6. Questions

1. What is the uncertainty in volume, using the micropipetters? So, for a 4.00 mL measurement, what is the uncertainty associated with that measurement?

2. Why do you have to blank the spectrophotometer every time you change the wavelength?

3. Why isn’t the calculation of the error in the mole percent done by finding the intersection of the highest possible positive slope line and the highest possible negative slope line?

4. (The hard question) Why do you use absorbance (A) instead of percent transmittance (%T) in this experiment? After all, it is difficult to get the same degree of precision using the A scale as you would on the %T scale. Hint: see appendix three in the textbook; which of the two quantities is actually proportional to the concentration of the complex, and therefore the number of moles of iron ion and ligand?

Part 7. Conclusion

The conclusion should be a brief recap of your results and sources of error (confidence you have in your results).

First sentence: State the number of 1,10-phenanthroline ligands the iron (II) ion has, along with the error.

Next couple of sentences: Compare your value to the actual number of ligands on the iron-phenanthroline complex. Suggest the major source(s) of error in this experiment and determine if these errors were of a random or systematic nature. If systematic, suggest a way to fix the error for the next group who uses this procedure.

Last sentence: How confident are you in your results? In other words, was your technique and result good enough that you would feel this procedure could be given to the next group of students without modification? If you do not feel this way, suggest how the procedure could be modified to prevent some of the problems you may have encountered.

Abstract

An abstract is a summary of a larger report or manuscript. In the sciences, it is used to index millions of articles; for instance, the Chemical Abstracts Service (CAS) indexes various articles by the chemical formula of the compound(s) used, so it is crucial that one writes the chemical names in the abstract of the journal article.

For this class, I am looking for a short (less than 100 word) summary of the major result(s) of your experiment, and the method by which you achieved this result.

All abstracts should have the name of the report author (you) and the author’s institutional affiliation (in this case, North Seattle Community College).

Specifically, for this experiment, use wording such as “We determined the number of phenanthroline ligands on the iron (II) ion to be ____________ +_________/-_________, through the use of visible wavelength spectroscopy on solutions of the complex. This result (was/was not) consistent with the true value of ___ ligands per iron ion.”

Note that the abstract really does cut to the chase and is even shorter than the conclusion.

Also note that, for lab #2, I will not supply “the fill in the blanks” wording!