Homework answers / question archive / Laboratory 3 Gravimetric Determination of Chloride in a Soluble Sample   Objective This experiment is performed to determine the percent chloride in an unknown soluble sample using gravimetric methods

Laboratory 3 Gravimetric Determination of Chloride in a Soluble Sample   Objective This experiment is performed to determine the percent chloride in an unknown soluble sample using gravimetric methods

Chemistry

Share With

---

Laboratory 3

Gravimetric Determination of Chloride in a

Soluble Sample

 

Objective

This experiment is performed to determine the percent chloride in an unknown soluble sample using gravimetric methods.

Background

The work must be shared in order to complete the lab in the allotted time. 

References: CDROM: 37B1 

                    Analytical Text: Chapter 12 

Chemicals Required

Week 1: Concentrated nitric acid 6 M ammonium hydroxide 0.1 M silver nitrate 6 M nitric acid RO water Unknown sample Equipment Required	Week 2: 6 M nitric acid RO water Concentrated hydrochloric acid
Week 1: 3-400 mL beakers 3-Stirring rods	Week 2: Rubber policeman Wash bottle

3-Watch glasses

3-Glass filtering crucibles  Vacuum filtration set up

 

Procedure

Week 1 

Cleaning Filtering Crucibles 

1 Set up and properly secure a Buchner flask. Place about 50 mL of RO water in the  flask. Turn on the fume vent over the filtration setup.

2. Place a crucible in a funnel and insert into the Buchner flask. 
3. Add about 5 mL of concentrated nitric acid to the glass filtering crucible and let  stand for 5 minutes. Open the vacuum tap and vacuum filter the nitric acid into the  Buchner funnel. 
4. Fill the filtering crucible with water and filter using vacuum. Repeat 2 more times  and break the suction by turning off the vacuum. 
5. Add about 5 mL of 6 M NH₄OH to the crucible, let stand a few minutes and vacuum filter. 
6. Fill the crucible with RO water and filter using vacuum. Repeat 6 - 8 times. 
7. Repeat steps 2 – 6 for the other 2 crucibles. 
8. Mark the crucibles with numbers 1, 2, and 3, place into a large beaker and dry in the oven at 105°C until the end of the lab period.
9. Just before the end of the lab period, remove the crucibles from the oven, and place them into the desiccator until your next class.

NOTE: Do not touch the crucibles with your fingers after this. Handle only with tongs or kimwipes.

           

Sample Precipitation 

1. Your instructor or technologist will provide you with your unknown sample (which has been dried by the lab technologist).  Immediately record in your lab notebook the number of your unknown sample.
2. Add 7 – 8 mL of 6 M nitric acid to about 300 mL of RO water.
3. Measure three samples of 0.15 - 0.20 g of unknown into 400 mL beakers by weighing on the analytical balance.  Weigh the samples to the nearest 0.0001 g.
4. Use 100 mL of the diluted nitric acid (from step 2) to dissolve each sample.
5. Slowly, while stirring, add 45 mL of 0.1 M AgNO₃ (using a graduated cylinder) to each solution.
6. Heat almost to boiling and digest (without boiling) the solutions for about 10  minutes. 
7. Check each solution by adding a few drops of AgNO₃. If a white cloud forms, add another 5 mL of AgNO₃ and digest another 10 minutes. Repeat until no white cloud forms when AgNO₃ is added. 
8. Let the beakers cool.  Cover the beakers using Parafilm and store in your locker until the next lab period. 

 

Week 2 

Filtration and Drying 

1. Retrieve your crucibles from the desiccator and weigh them on an analytical balance. Weigh the crucibles to the nearest 0.0001 g.
2. Set up a vacuum filtration apparatus.
3. For each beaker containing AgCl precipitate, decant the supernatant liquid through the weighed filtering crucible.
4. Make a dilute solution of nitric acid by adding about 2 mL of 6M HNO₃ to 500 mL of RO water. 
5. Wash each precipitate (while it is still in the beaker) using 60 – 80 mL of the diluted nitric acid.  Decant the precipitate washings through the filtering crucible.  Do this 3 or 4 times for each precipitate.
6. Quantitatively transfer the AgCl into the crucibles using your wash bottle. Use a rubber policeman to scrape any particles adhering to the walls of the beaker.
7. Continue washing the precipitate in the crucible with diluted HNO₃ until the filtrate is free of Ag⁺. To test the washings for Ag⁺, collect a small volume from the filtration flask in a small beaker and add a few drops of HCI. Dump out the rest of the liquid in the filtration flask.  If no cloudy precipitate forms, no Ag⁺ is present. 
8. Repeat steps 3 to 7 for your other 2 samples
9. Dry the precipitates at 105°C for at least 1 hour.
10. Store the crucibles in a desiccator until cool.
11. Weigh the crucibles and their contents to the nearest 0.0001 g. 
12. Calculate the % Cl^- in your sample. 

 

Notes 

1. A slightly acidic solution is required for selective precipitation of AgCl. 
2. A slight excess of silver will decrease the solubility of AgCl.
3. Photo decomposition of finely divided silver can occur and produce Ag and Cl₂, leading to low results for your chloride determination. If you see a violet colour in your precipitate, elemental silver is present and  an additional reaction can occur, producing AgCl and ClO₃, causing  high results. These effects can be reduced to a minimum by avoiding exposure of your precipitate to high levels of light. Efficient performance of your precipitation will also reduce the production of elemental silver. Your product should be stored in your locker (in the dark) until weighed. 
4. Use only the amount of silver nitrate that you need and return any unused silver nitrate. 
5. Use a separate stirring rod for each sample.            

Lab Report Format

 

This lab report requires:

 

COVER SHEET

TITLE

PURPOSE 

OBSERVATIONS

CALCULATIONS

RESULTS

CONCLUSION

POSSIBLE SOURCES OF ERROR

REFERENCES

PHOTOCOPY OF ALL DATA FROM LAB NOTEBOOK

 

COVER SHEET

The cover sheet is to be completely filled in, attached to the report and placed in the appropriate folder on the front bench.  Sample number, mean, standard deviation and % relative standard deviation must appear on the cover sheet or a mark of 0 for accuracy and precision will be given.

 

TITLE

The title should be the same as that in your lab manual.  

 

PURPOSE

The purpose should be 1 or 2 short sentences describing the purpose of your lab.

 

OBSERVATIONS

This section include both physical and numerical observations.

Physical observations may be written in sentence format or in a table.  Each week observations should be a separate paragraph or in separate tables.

Numerical observations are to be in table format.  The table must have an appropriate title and heading.

The observed data must include the following: weight of unknown sample, at least two weight readings for cleaned, empty crucible, and at least two weight readings for crucible with precipitate.

 

CALCULATIONS

As in previous labs, a sample calculation should be shown for the first sample.  Mean, standard deviation and % relative standard deviation are to be calculated.

 

RESULTS

The results of your calculations are to be summarized into a table.  The results table should show the three individual results for % chloride, the average % chloride, the standard deviation, and the % relative standard deviation.

 

CONCLUSION

The Conclusion is a sentence stating “Sample number ___ was found to contain ___ % chloride with a relative standard deviation of ____ %.

 

SOURCES OF ERROR

List at least 3 sources of error.  Describe the type of error and whether it will increase or decrease the % Cl^- in your final answer.

 

REFERENCES

List all references used in your report.  Remember your text, class notes and lab techniques notes are all applicable to your lab.  All web sites must be easily accessed by the instructor. 

Check to make sure they are accurate and can be easily accessed using your reference. 

Marks are deducted for poor references.