15 Nov 2010

Synthesis and characterization of 12-tungstosilicic acid - a modification

Lab Experiment

Submitted by Randall Hicks, Wheaton College
This lab experiment is an update on the synthesis of the title acid. An account of this synthesis can be found in William Jolly's book, The Synthesis and Characterization of Inorganic Compounds. The synthetic procedure is reliable and was retained. However, I modified some content and added components to extend to a two-week experiment. This lab now covers the synthesis, an acid-base titration with error analysis, and the use of TGA to complement the titration results.
Learning Goals: 

A student performing this experiment should:

1. gain proficiency in chemical synthesis;

2.  use error propagation methods to state titration results with an appropriate uncertainty; and

3. interpret TGA data for dehydration.

Equipment needs: 
Normal lab glassware and equipment (round bottom flasks, separatory funnels, stirring hot plates, etc.), a low temperature oven for drying the synthesized compound, analytical balance, thermogravimetric analyzer (TGA).
Implementation Notes: 

Jolly's synthesis, while reliable, yields 32 g of product, which is more than enough. I scaled back the quantities used to 25% of what was called for. Students should expect a yield of 8 g.

I purposely added in the error propagation as a connection to the analytical chemistry course that all students had previously taken. This could be omitted if your students have not had this preparation. If you do the error analysis, you will need to standardize the NaOH and report its concentration with uncertainty, so that students can use this in their calculations. Sample titration calculations and results are included in the Excel file.

If time permits and you are so inclined, you could acquire a TGA profile for each student's sample. In this experiment, they should all be about the same and obtaining a profile for each was deemed to be a waste of resources. Therefore, I instead demonstrated the sample prep and operation of the instrument to the students and used the same data for the entire class. If you do not have access to a TGA, I have attached a text file of the raw data which you can use to generate the TGA profile (plot). The data begins on about line 40, after all the instrument parameters. Columns two and three contain the pertinent data of temperature and mass, respectively. A second Excel file contains the worked up TGA data and the plot.

Due to the drying period, this lab is conducted over two weeks. However, neither week's activities completely use allocated time. You could maximize efficiency by having students work on some aspect of a different lab experiment during one or both weeks.


Time Required: 
Two lab periods (see comment in 'implementation notes')
Evaluation Methods: 
Evaluation was determined by the quality of the lab report. In addition to the usual components of the report, several questions- which relate this lab to previously covered course content- were posed at the end as well. So I was specifically interested to see if students could recall the info and make these connections. For example, could they do the errror propagation correctly to yield a resonable uncertainty for their result? Could they recall molecular symmetry principles to determine the number of unique positions in the Keggin ion? TGA was used in a previous lab, so I was hoping that the students could independently analyze the data. 
Evaluation Results: 
The quality of reports across the class was varied, as is the case for most lab reports. While I had hoped that students could perform the error analysis and TGA analysis independently, many of them needed a refresher, after which they could perform these tasks. In some sense, the purpose was realized as I wanted to further cement previously learned materials via this experiment.
Creative Commons License: 
Creative Commons Licence


Dar Sir/Madam,


   I would like to know how to determinte the percentage yield of the product. As the equaton is unbalanced, how can you calculate the percenage yield? Can you show me the steps to calculate?


Best Regards
Wong TM






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