posted by Gloria .
Below i wrote a few problems, i did them all but in the book i don't have results, so i don't know if i did them right or not. I would really appreciate if someone would go through the problems and tell me if i got the right results. If i made a mistake somewhere please let me know and i will try to do the problem again.
1. After appropriate preliminary treatment, the thalium in a 10.2 g pesticide sample was precipitated as TlI and was weighted as such. Calculate the percentage of Tl2SO4 (fw=504.8) in the sample if 0.1964 g of TlI (fw=331.27) was recovered.
MY RESULT: 1.47%
2. A 5 g sample of a pesticide was decomposed with metallic sodium in alcohol and the liberated chloride ion was precipitated as AgCl. Express the results of this analysis in terms of the percentage of C14H9Cl5 (fw=354.5) based upon the recovery of 0.1606 g of AgCl.
MY RESULT: 1.59%
3. The nitrobenzene (fw=123) in a 0.739 g sample was determined by dilution with an Hcl/methanol mixture, followed by the introduction of 0.465 g of pure tin. The mixture was refluxed for an hour, during which the nitrobenzene was reduced to aniline:
2C6H5NO2 + 3Sn + 12H -> 2C6H5NH2 + 4H2O + 3Sn
When the reaction was complete the unused tin was isolated by filtration, dried and found to weigh 0.128 g. Calculate the percentage of nitrobenzene in the sample.
MY RESULT: 31.5%
4. Titration of the I2 produced form 0.1238 g of primary-standard KIO3 required 41.27mL of sodium thiosulfate. Calculate the concentration of the Na2S2O3 solution.
MY RESULT: 0.08412 M
5. A 0.1884 g sample of impure Na2Co3 required 31.56 mL of 0.1056 M HCl. Calculate the percent purity of the Na2Co3.
MY RESULT: 0.02216 M
6. The sulfur in a 0.5073 g organic sample was burned in a steam of O2, the combustion products were bubbled through H2O2 to convert SO2 to H2SO4. Titration of the H2So4 required 33.29 Ml of 0.1115 M NaOH. Calculate the percent S in the sample.
MY RESULT: 11.73%
7. Exactly 40Ml of a solution of HClO4 was added to a solution containing 0.4793 g of primary-standard-grade Na2Co3. The solution was boiled to remove CO2 and the excess HClO4 was back-titrated with 8.7 mL of a NaOH solution. In a separate experiment, 25 mL of NaOH neutralized 27.43 mL of HClO4. Calculate the molarity of the acid and the base.
MY RESULT: acid-0.2969M, base-0.3258
8. A 0.3396 g sample that assayed 96.4% Na2SO4 was titrated with a solution of BaCl2. What is the molarity of the Bacl2 solution if the end point was observed when 35.7 mL of the reagent was added?
MY RESULT: 0.06456 M
9. The thiourea in a 1.455 g sample of organic material was extracted into a dilute H2SO4 solution and titrated with 37.31 mL of 0.009372 M Hg(2+) via the reaction
4(NH2)2CS + Hg -> [(NH2)2CS]4Hg
Calculate the percent (NH2)2CS in the sample.
MY RESULT: 7.51%
For #4 I obtained 0.0841054 which I would round to .08410 M. I used 214.00 for the molar mass of KIO3.
#5 doesn't ask for molarity. It asks for % Na2CO3 in the original sample. I obtained in the neighborhood of 94% or so.
For #7 I obtained 0.2970 M for the acid and 0.32587 which I would round to 0.3259 for the base. I used 105.989 for the molar mass of Na2CO3.
For #9 I obtained 7.3175% which I would round to 7.318%. Aren't you allowed 4 s.f.? I used 76.122 for the molar mass of (NH2)2CS.
A note: Ml is confusing for mL. I encourage you to use mL for milliliters as you did in problem 8.
In the future I would encourage you to post these one at a time AND show your work on the problem. I know that takes a lot of your time to type each one but think how much time I spent working these nine problems and comparing my answers with yours. If you had shown your work I would have been able to check each one in 1/2 the time or less. This way I've spent an hour helping one student; the other way I could have helped more. However, the board is not busy at the moment and I spent the time because I know you are a repeat user of this forum and I have the time now. When we get busier in the school year I won't have the time and these long posts with no work shown gets left at the starting gate.
These are some basic problems you've worked and you appear to have a good grasp of the concepts.
First of all, thank you so much for answering and i'm sorry for writting all the problems at once. I won't do that anymore in the future.
#5 i was looking at the different problem with my result, that's why i wrote the concentration :S
#9 thank you, i just realized that i miscalculated the molar mass.
Thank you for your help and once again i apologize.
Another thing you can do is to show the molar mass you use in the calculations. If you use one molar mass and I use a different value we will never arrive at the same answer although both of us worked the problem correctly. That's why I showed the value of the molar mass I used when I didn't agree with your answer.