Chem 1121 - Robertson

Moderator: Ron Robertson

Re: Chem 1121 - Robertson

Postby Ron Robertson » Wed Apr 08, 2015 7:30 pm

Determining the Identity of an Unknown Weak Acid

Pre-lab problem #5 answer - the pH is 3.1

Tomorrow's lab is our 2nd and final long writeup. I will have an outline of what needs to go into the report. It will be due on Wednesday at 5 pm in an electronic version on Turnitin.com and a paper copy is due in the box outside my door at the same time. No other information is required. The lab report is worth 40 pts.

For tomorrow's quiz - Make sure you review all calculations from last week and understand what we are trying to do tomorrow.

Probably the last extra credit opportunity is next Friday at the university Research forum in the ballroom at the MUC. Extra credit can be earned by evaluating 5 projects similar to the MTSEF assignment. The posters should be up from 1 - 4:30 pm. You will evaluate 5 different projects for a max of 10 pts. The evaluation sheet can be found at http://apbrwww5.apsu.edu/robertsonr/nova/xtracredit.pdf. See the announcement flyers up in the department for more details. There are oral presentations as well as the posters.
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Fri Apr 10, 2015 10:08 am

By now you should be trying to find that inflection point that is crucial to your identification of the weak acid by MM and pKa.

The lab explains how to do this manually after printing the graph itself. Excel or Logger Pro can be used with this method.

If you want to try to do the entire thing without manually drawing lines using logger Pro here is what you need to do.

A. Find the steeply rising part of your curve and use a "mouse hover" over an estimated middle range of the volume added in this section to find the pH values on the lower linear line and upper linear line at this point. Take the average of these pH values and use the "mouse hover" again to determine the volume that corresponds on your curve to this pH. Take the volume to 0.01 mL.

B. Use the "Examine" function under "Analyze" on the menu to show a vertical bar at this volume. Use "Text Annotation" on the "Insert" menu to label this point (with volume) on your graph as the inflection point.

C. Take 1/2 of this volume and find the pH on your linear fit line that corresponds to this volume (using the "mouse hover" technique again). This pH is the pKa of your acid. It would be nice to label this with a text annotation as well although not shown in the lab.

I would be glad to work with you on this.
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Mon Apr 13, 2015 9:46 am

The Buffer quizzes are graded and in the box for pickup. A key is on the board. Come on by and let's talk about what you missed.
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Wed Apr 15, 2015 10:09 am

Wow, thermodynamics and acid/base equilibria on one quiz, and another great graph for the thermo lab! It doesn't get much better.

Make sure you know how to do the prelab for the quiz. I am also going to give you a freebie question that is on the quiz. Since I am giving you the question I can't give you the answer before the quiz even if you ask. If you don't know the answer get with others and see if you can determine it. It is very similar to what we did on the last experiment but I have switched the identities of the acid and base (from strong to weak).

"Sketch the graph of pH vs acid added for a titration of a weak base with a strong acid. Then explain how this graph could be used to determine the pKb of the weak base."

You will need to submit by 5 pm on Monday. . .data sheets pages 5 and 6 of 9, a revised sample calculations page, your graph, and post lab questions 1, 2, and 3 (page 8 of 9)

See you tomorrow (if not before). . .
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Wed Apr 15, 2015 8:00 pm

Answers to #3 on the pre-lab

slope = -5004 K
Intercept = 19.04
enthalpy change = 41.6 kJ
entropy change = 158 J/K
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Tue Apr 21, 2015 3:59 pm

Finished grading your Unknown Weak Acid lab reports

The reports are marked and are in the box for pick-up. The distribution was 6 A, 5 B, 2 C, 5 D, 2 F. Grades ranged from 40/40 to 17/40 - we had a wide range of quality of work. Honestly I think this is about where our class is on the understanding of the lab/lecture material. We have many good to very good students but we also have several weak students. The amount of work was evident in your reports and I very much appreciate it. Writing good science requires a good understanding of the ideas and principles of the lab work as well as an ability to communicate it with the written word. This is no easy task. I am still evaluating the originality reports and the late submissions by a few which may require very small point changes.

I would enjoy talking with you about your reports and clearing up some problems in understanding as well as writing. One problem with some of your reports was the lack of an unknown ID number for your acid. Many did not adequately explain how you actually obtained the inflection point and the pH for the pKa from the graph itself. The concept was there but not how you actually did it. The major design error with the lab was probably finding the inflection point itself. One student dug deeply into the analysis and found how to get a 2nd derivative plot from Logger Pro which very nicely identifies this crucial point. The overall data you obtained and the identification of the acid (for those that gave me an ID) was remarkably good - some of the best work I have ever seen in all my years of supervising this lab. Many of you have picked up the Logger Pro program quickly and seem to be enjoying its power and ease of use.

Now I'm off to grade those lab sheets on last week's lab. You will be hearing from me as soon as I complete the grading. One last lab on electrochemistry. Let's finish strong with no regrets!
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Tue Apr 21, 2015 6:45 pm

Your lab sheets for the thermo lab are graded and ready for pick-up as well as last week's quiz. A key is posted. With several different sets of papers for you to get, I have changed the holder for the papers to a divided file.

You handled this last thermo experiment pretty well.
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Wed Apr 22, 2015 7:39 pm

The last lab - pretty easy compared to what we used to do with electrochemistry

Activity Series Lab Write-up, Due Monday, April 27 at 5 pm

1. A procedure section as you would write a lab report. (typed)

2. Data Sheet p.2 of 3 (typed)

3. A discussion section as you would write a lab report. (typed)

In the first paragraph answer the purpose by determining your activity series, explaining your reasoning from your data sheet. Then compare your ranking to a predicted ranking from a table of electrochemical potentials (use the “Reduction Half Equation” handout as an attachment to your report), again explaining your reasoning.

In the second paragraph discuss errors, avoidable and unavoidable. Don’t invent imaginary errors but give a realistic assessment of what things could have affected your results.

In the third paragraph give some practical applications of knowing the activity series for these metals.
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Sat Apr 25, 2015 3:46 pm

I have had a few questions on Friday as well as today by email about the use of the reduction potential table to get the activity series of the elements. I have replied to one student in the following way.

You already have a table of reduction potentials that I gave you. If you reverse those to obtain oxidation potentials and rank them from most positive to least positive you will have a ranking of the activity of the metals. Activity means how reactive that element is versus another element. To prove this you can take the oxidation potential of any element on your list and add to it the reduction potential of an element underneath it. The sum will be a positive number indicating a spontaneous reaction.

I have asked you to explain your ranking of activity of the different metals based first on the reactions you did in lab. Then I want you to explain how that ranking could be predicted from a table of reduction or oxidation potentials. The ranking should be the same.

Stop by on Monday for more help. If you do this you will see the practical use of your lecture material on reduction potentials.
Ron Robertson
 
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Re: Chem 1121 - Robertson

Postby Ron Robertson » Wed Apr 29, 2015 10:41 am

Important deadline

In working on your grades I have noticed that a few of you have not completed some of your online OWL assignments. You have until tomorrow (April 30) at 5 pm to complete those assignments and then I can complete your grades.
Ron Robertson
 
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