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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: ecestudent on April 15, 2010, 03:54:06 PM

Title: Determination of an Unknown Salt !
Post by: ecestudent on April 15, 2010, 03:54:06 PM: For this lab I was given an unknown salt and I so far I have done precipitation tests, found the pH, and found out it decomposes with heat when I did a waters of hydration lab and after about 8 min my unknown was gone.

So far I have:

Flame test seemed like the cation was sodium but I am going to re-do this again tomorrow, it didnt seem to be "enough" orange flame, I then used colbalt glass and didnt see anything. But I want to double check.

pH= 6.02 so it must be a neutral salt

UK+ NaOH= no precipitate
UK+ Na3PO4= no precipitate
UK+ Na2CO3= No precipitate
so it must be a first row cation
*and when I added a few drops of HCl to the unknown gas was evolved as it seemed, but there were no bubbles. it was just a small amount of white looking smoke. I am not sure what to make of it.

then:

UK+ AgNO3= milky white precipitate
UK+ Pb(NO3)2= white precipitate

so far I think the cation is Na+, which is why I want to make sure with another flame test because all the first rows have great color, but the range of anions is still wide.

I am not sure what to conclude or what other tests to do, anyone have any suggestions on possible cations/anions and tests I should do? any help is appreciated!
Title: Re: Determination of an Unknown Salt !
Post by: Borek on April 15, 2010, 05:22:29 PM: Quote from: ecestudent on April 15, 2010, 03:54:06 PM
UK+ AgNO3= milky white precipitate
UK+ Pb(NO3)2= white precipitate

Quote
the range of anions is still wide.

Really?
Title: Re: Determination of an Unknown Salt !
Post by: ecestudent on April 15, 2010, 06:21:08 PM: Cl-, I-, Br-, or a sulfate, maybe a fluoride. ok so maybe not soo many. but I'd rather narrow it down more. I still dont even have my cation all the way down yet. just only that it is a first row
Title: Re: Determination of an Unknown Salt !
Post by: UG on April 15, 2010, 06:38:37 PM: If you think it is a sulfate, try adding BaCl₂ to it and see if there is a precipitate.
Just my own two cents, it's probably either a chloride or a bromide. Try adding dilute ammonia to the precipitate you got from adding AgNO₃, if it is AgCl, it will dissolve.
Title: Re: Determination of an Unknown Salt !
Post by: ecestudent on April 15, 2010, 06:43:50 PM: I thought about using BaCl₂ but I couldnt find any in the stock room. Im going to look again but thanks for the second suggestion, Im going to try that
Title: Re: Determination of an Unknown Salt !
Post by: Borek on April 15, 2010, 06:45:49 PM: Quote from: ecestudent on April 15, 2010, 06:21:08 PM
Cl-, I-, Br-

Both precipitates were white?
Title: Re: Determination of an Unknown Salt !
Post by: ecestudent on April 15, 2010, 06:53:46 PM: yup, what are you getting at?
Title: Re: Determination of an Unknown Salt !
Post by: UG on April 15, 2010, 07:12:52 PM: Lead iodide is yellow.
Title: Re: Determination of an Unknown Salt !
Post by: ecestudent on April 15, 2010, 07:17:04 PM: ok so I^- is out, thanks
Title: Re: Determination of an Unknown Salt !
Post by: skyjumper on April 15, 2010, 08:50:20 PM: Lead bromide melts at about 367 to form a clear liquid, while lead chloride melts at +500 degrees, try a melting point test on the lead precip
Title: Re: Determination of an Unknown Salt !
Post by: Borek on April 16, 2010, 02:48:27 AM: Quote from: skyjumper on April 15, 2010, 08:50:20 PM
Lead bromide melts at about 367 to form a clear liquid, while lead chloride melts at +500 degrees, try a melting point test on the lead precip

Checking if the precipitate dissolves in a hot solution is much easier.
Title: Re: Determination of an Unknown Salt !
Post by: NaOH on April 16, 2010, 01:03:12 PM: Maybe its ammonium chloride, on class I made a reaction of ammonium chloride solution and silver nitrate solution and it gave a white percipitate.