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| | From:  HaSS__Baby (Original Message) | Sent: 6/10/2004 5:01 AM |
I NEED TO KNOW WHAT IONS DOES THIS UNKOWN HAVE , DEPENDING ON THE LAB RESULTS I GOT .. THANKS Unknown B / Procedure : Part A - Anion Anlysis - SO4 m CO3,Cl,I PROCEDURE 1+2+3 : 1.0 mL OF UNKNOWN + 5 drops of 6.0 mol/L NH4OH ---> ORANGE PRECIPITATE PROCEDURE 4 : Centrifuging and decanting the solution into a clean test tube. Discarding the precipitate. adding 20 drops of 0.20 mol/L Ba(NO3)2 ----> DARK ORANGE PRECIPITATE PROCEDURE 5 : WASHING THE PRECIPTATE WITH 20 DROPS OF DISTELLED WATER THEN CENTRIFUGING . DECANTING AND DISCARDING THE WASH SOLTUION. PROCEDURE 6: To the ORANGE PRECIPITATE , 10 drops of the orange of 6.0 mol/L HNO3 ---------> LIGHT MILKY SOLUTION / PRECIPITATE DISAPPEARS. PROCEDURE 7 :SOLUTION FROM PROCEDURE 4 + 20 DROPS OF 0.10 mol/L AgNO3 ---------> WHITE PRECIPITATE unkown a : light pink , al , CO, co3 unknown b clear , so4, cl, PROCEDURE 8: centrifuging , decanting and discarding the supernatant liquid. washing the precipitate with 10 drops of distelled water. decanting and discarding the wash solution PROCEDURE 9: adding 20 drops of 6.0 mol/l NH4OH to precipitate and stirring PROCEDURE 10 :centrifuging, and seperating the solution from the REMAINING SMALL WHITE PRECIPITATE PROCEDURE 11: TO THE SOLUTION FROM PROCEDURE 10, 20 DROPS OF 6.0 HNO3 -----> LIGHT WHITE PRECIPITATE |
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| | | From: ·Steve· | Sent: 6/10/2004 10:12 PM |
Hi, in Procedures 1-3 I was confused about the ions you are testing for - Unknown B may contain one or more of the following ions: sulfate, carbonate, chloride, or iodide ion, is that right? However, NH4OH will not react with any of these, so I suspect that the orange precipitate was the insoluble hydroxide of a metal cation instead. Ammonium hydroxide (aqueous ammonia) is a weak base and, when added to a solution containing Fe3+ ions, for example, will immediately form a rust-colored ("orangish" when very fine) precipitate of Fe(OH)3(s).
Are you testing for any metal cations such as Fe3+ also, or was Fe2+ or Fe3+ ion added earlier? Possibly somewhere in Procedures 1-3 ferrous or ferric ion was added to the unknown in order to selectively precipitate carbonate ion, if present, and then 6 M NH4OH was added to remove excess Fe2+ or Fe3+ ion (as insoluble Fe(OH)2 or Fe(OH)3). Sulfate, chloride, and iodide ions will not form an insoluble precipitate with Fe2+ or Fe3+ ion. But I confess I'm not sure how effective ferric ion is at precipitating carbonate ion, because, since carbonate ion is a weak base, Fe(OH)3(s) could form instead, making the test uncertain.
Procedure 4 looks OK - adding Ba2+ ion will form insoluble precipiates with sulfate ion and carbonate ion and this is a common way of checking for these and precipitating them out. But, BaSO4(s) and BaCO3(s) are both insoluble white solids; a dark orange precipitate would not form. Further tests on the precipitate are needed in order tell which ion, sulfate or carbonate (or both) is in the precipitate.
Procedure 6 - Was this test on the first "orange precipitate" or the second "dark orange precipitate"? If the first orange precipitate was Fe(OH)3(s), it would dissolve upon addition of HNO3 forming a clear yellow or orange solution (Fe(NO3)3(aq) ), depending on how concentrated the solution is. Assuming the second dark orange precipitate is BaSO4 and/or BaCO3 contaminated with Fe(OH)3 perhaps, making it dark orange, addition of HNO3 would immediately dissolve the BaCO3 giving obvious bubbling of CO2 gas. BaSO4 would not dissolve. So, if you observed this bubbling, you can be sure your precipitate was BaCO3 and therefore carbonate ion was present.
Procedure 7 - The solution could contain chloride ion or iodide ion or possibly both, I'm assuming. Chloride ion gives a white precipitate of AgCl(s) when AgNO3 solution is added, while AgI(s) is light yellow in color. Looks like your unknown had chloride ion.
Procedure 9 - Is this test on the precipitate from Procedure 7? If it is, adding ammonia solution (NH4OH) should dissolve the silver precipitate, forming the Ag(NH3)2+ ion which is soluble.
Procedure 10 - I'm not sure what this "remaining small white precipitate" is. Insoluble silver compounds normally dissolve when ammonia is added, forming the Ag(NH3)2+ ion. Perhaps this solid is just some excess AgCl that didn't dissolve.
Procedure 11 - Adding HNO3 to the Ag(NH3)2+ solution converts the NH3 to NH4+ ion, allowing the Ag+ ions to recombine with the Cl- ions (which are still present), reforming AgCl(s). So, this is just a confirmatory test for the AgCl.
Hope this helps, in spite of the uncertainties!
Steve
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Reply
| | | From: jimfee | Sent: 6/11/2004 11:24 AM |
Steve or any1 in the group. I need immediate help w/two questions regarding
DNA. I guess a little off subject, but if u, Steve or any1 can help me it
would be greatly appreciated. Here are the questions:
1. What do you think was the most significant expertiment in the discovery
of the structure of DNA? Why?
2. How do you think the race to discover the structure of DNA would be
different if it was happening today? Base your answer on scientific,
political and cultural differences between the time period of the book and
now.
*The book is titled: "THE DOUBLE HELIX" by James D. Watson
Plz help me out if u can. Thank u in advance 4 ur assistance.
Jimfee
>From: "·Steve·" <[email protected]>
>Reply-To: "Chemistry Corner" <[email protected]>
>To: "Chemistry Corner" <[email protected]>
>Subject: Re: HELP TO FIND OUT WHAT IONS IN THIS UNKOWN
>Date: Thu, 10 Jun 2004 14:12:32 -0700
>
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Reply
| | | From: ·Steve· | Sent: 6/11/2004 8:48 PM |
There are some good articles about the discovery of the DNA structure at the Nature web site at http://www.nature.com/nature/dna50/ which focuses on the "double helix: 50 years of DNA".
A good candidate for "the most significant expertiment in the discovery of the structure of DNA" would be Dr. Rosalind Franklin's X-ray diffraction photograph of DNA which provided the crucial experimental evidence for the helical structure of DNA that Watson and Crick based their double helix model on. Here is the abstract of the article, "The double helix and the 'wronged heroine'" from the Nature site:
In 1962, James Watson, Francis Crick and Maurice Wilkins received the Nobel prize for the discovery of the structure of DNA. Notably absent from the podium was Rosalind Franklin, whose X-ray photographs of DNA contributed directly to the discovery of the double helix. Franklin's premature death, combined with misogynist treatment by the male scientific establishment, cast her as a feminist icon. This myth overshadowed her intellectual strength and independence both as a scientist and as an individual.
A quote from the article states:
"Watson and Crick seem never to have told Franklin directly what they subsequently have said from public platforms long after her death �?that they could not have discovered the double helix of DNA in the early months of 1953 without her work."
The article notes that there are a number of reasons why Dr. Franklin's did not receive immediate credit or acclaim for her contribution to the discovery of the structure of DNA; for example, she did not consider herself to be in any kind of "race" to discover the structure, but certainly in today's world scientific community many of the cultural barriers that existed in 1953 are much less in force. For instance, science is not such a male-dominated field now as is was then. In those days it would have been much harder for a woman to be accepted into the "club" on the same intellectual and information-sharing footing that male scientists enjoyed. Of course it cannot be said that these barriers are gone in today's world, but much progress has been made in lowering those barriers.
That is not to say that petty competitive ambitions and quests for scientific glory and/or profit are not any less now than then, however! See for example this article, "Aids: will the true story ever be told?" found at http://www.millennium-debate.org/ind8mar023.htm. Here is a quote:
"the truth is that the bitter dispute over who was the first to discover HIV and link it to Aids was marred from the start by a lack of co-operation, the suppression of data, and a reluctance to share anything that could give the opposition a competitive advantage. A new book published in the US called Science Fictions describes the 20-year history of the race to find the cause of Aids.
It is a story that shames science, not least because it involved an alleged cover-up that reached to the highest levels of the US scientific establishment."
I guess though times change, some things always stay the same!
I've attached a copy of the Nature article to this message. It is in pdf format and allowed to be freely distributed.
Steve
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| | nature01399.pdf |
Reply
| | | From: jimfee | Sent: 6/12/2004 4:27 AM |
Steve -
Thank u so much. That was a big help 2 me. It worked out perfectly. Thank
u once again 4 all of ur help.
Jimfee
>From: "·Steve·" <[email protected]>
>Reply-To: "Chemistry Corner" <[email protected]>
>To: "Chemistry Corner" <[email protected]>
>Subject: Re: HELP TO FIND OUT WHAT IONS IN THIS UNKOWN
>Date: Fri, 11 Jun 2004 12:48:55 -0700
>
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thanks alot u really helped and i got 85%  |
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Reply
| | | From: ·Steve· | Sent: 6/13/2004 4:39 PM |
Good deal! Sometimes those qualitative tests are maddenly uncertain - was it a positive test, or was that really a negative result with a bit of an impurity giving what appears to be a positive test? Is that itty bit of brownish stuff what the lab manual says is the orange precipitate, or is it something else? Doing these tests is a skill that requires practice, experience, and sometimes plain ol' luck! Steve |
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