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budullewraagh

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Qualitative Tests
« on: December 14, 2004, 06:41:34 PM »
i am a member of a competitive team of physical and natural scientists at my high school.  we compete in various events and i serve as the chemist.  one of our events involves qualitative analysis, which isn't terribly difficult usually, but given the restrictions they put on us, it appears to be quite difficult.

if you could give me feedback on my work and help me with things i am missing, i would be greatly appreciative.

equipment/reagents available:
-test tubes, stirring rods
-deionized water
-any ONE of the following: 3M HCl, 3M NaOH, 0.1M AgNO3 or phenolphthalein

we will have to identify any number of the following solids:
-NaHCO3, Na2SO3, Mg(OH)2, CaCO3, NaH2PO4*2H2O, NH4Cl, ZnCl2, KI, KOH, Al(NO3)3*9H2O

CaCO3 will be easy nomatter which reagent i choose, as it is insoluble in water

if i use the HCl, i will notice bubbling with CaCO3 and NaHCO3, and feel an exothermic reaction with Mg(OH)2 and KOH (and also Na2SO3??)

if i use the NaOH, i will notice a reaction with  NaH2PO4*2H2O, NH4Cl, Al(NO3)3, (ZnCl2 perhaps?)

if i use the AgNO3, i definitely will precipitate the halides, so count out NH4Cl and ZnCl2, KI, and i'm not sure but perhaps Na2SO3, Mg(OH)2, KOH

the phenolphthalein would indicate Mg(OH)2, KOH, and perhaps CaCO3?

so i think the silver nitrate would be the best reagent to use, although i may be wrong in its capabilities.  if i am, please let me know.  the problem is that nomatter which reagent i choose to use to aid in my analysis, i will have a lot of precipitates that will be less than easy to identify and a lot other unaffected chems.

i suppose i could use solubility, although i do not believe we will be given a balance and i know we are given only 10g of each of the solids.  thanks in advance

Demotivator

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Re:Qualitative Tests
« Reply #1 on: December 14, 2004, 09:44:20 PM »
You may be right about  silver nitrate.  (I looked at it briefly)
The beauty of it, though, is I think you can use the unknown solids (provided the list is exact) themselves as reagents and use combinations to deduce and elliminate possibilities.
For example,  KOH can be used as a base to react and precipitate Al(NO3)3*9H2O as the hydroxide.  NH4Cl can be usecd as acid to test for bicarbonate. NH4Cl reacts with base to produce a smell of ammonia, etc.
 

Offline AWK

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Re:Qualitative Tests
« Reply #2 on: December 15, 2004, 01:58:29 AM »
1. Two compounds are insoluble in water : Mg(OH)2 and CaCO3. They can be identified by reaction with HCL (bubbling or not)

2. ZnCl2 andAl(NO3)3*9H2O will produce white precipitates with NaOH (add dropwise), Zn(OH)2 will dissolve in excess of NaOH. THe other six compounds do not show specific reactions with NaOH

3. Test with acidification shows distinct bubbling only for NaHCO3.

4. Two compounds: KI and NaH2PO4*2H2O forms yellow precipitates with AgNO3. Silver phospate will change after treatment with HCl (yellow to white precipitate) where us AgI not.

5. KOH forms brown precipitate of Ag2O.

 6. The remaining two compounds form form white precipitates.
 If you reverse the reaction adding this compounds (Na2SO3 or NH4Cl) to a drop of AgNO3 you can observe that in one test tube white precipitate will dissolve in excess of reagent - this will work in the case of Na2SO3.
AWK

budullewraagh

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Re:Qualitative Tests
« Reply #3 on: December 15, 2004, 03:11:55 PM »
thanks for your help all.

my one problem is the fact that we are given EITHER HCl, NaOH, AgNO3, OR phenolphthalein.

only ONE of the above is given to us (our choice).

Offline jdurg

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Re:Qualitative Tests
« Reply #4 on: December 15, 2004, 03:38:04 PM »
thanks for your help all.

my one problem is the fact that we are given EITHER HCl, NaOH, AgNO3, OR phenolphthalein.

only ONE of the above is given to us (our choice).

Yes, but that doesn't mean that you can't take one of the solids you're investigating and use that solid, once you've figured it out, to test out the other solids.
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budullewraagh

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Re:Qualitative Tests
« Reply #5 on: December 15, 2004, 06:09:22 PM »
indeed, but AWK posted various tests that work with the use of various reagents that cannot be formed from reactions between the powders we get or our one reagent and a powder

Offline jdurg

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Re:Qualitative Tests
« Reply #6 on: December 15, 2004, 09:51:30 PM »
I think I would use the HCl.  Here's why.

1):  NaHCO3 and HCl will give a bubbling of carbon dioxide gas from your clear solution.

2):  Na2SO3 and HCl will give off an odor of rotted eggs which will be fairly noticeable.

3):  Mg(OH)2 and HCl will result in an insoluble solid dissolving in the HCl/water mixture and some heat given off.  So you'll go from an insoluble compound to a clear solution without the evolution of gas.

4):  CaCO3 is insoluble in water, but if you add HCl some CO2 will bubble off and the solid at the bottom of the beaker will dissolve.

5):  NaH2PO4*2H2O:  By process of elimination, this one will be the only salt that you haven't figured out.  (Once I can think of a good test, I'll add it in).

6):  NH4Cl:  Add your Determined KOH to this and it will give off an ammonia odor.

7):  ZnCl2:  Add some Sodium Sulfite and you'll ppt out ZnSO3.  Pour the aqueous part off into another empty test tube.  Add some HCl to this aqueous solution.  If nothing happens, then the compound is ZnCl2.  If some brown nitrogen oxides are given off, then you have the aluminum nitrate.  

8):  KI + HCl will result in the formation of HI vapor which will be very noticeable as the solution turns a red orange color.

9):  KOH will absorb water from the air and this will be readily observable.  (You'll be able to see the water in the collecting on the crystals/flakes.  Plus, the disolving of KOH gives off a good deal of heat).  

10):  Al(NO3)3*9H2O:  Add some Sodium Sulfite and you'll ppt out Al2(SO3)3.  Pour the aqueous part off into another empty test tube.  Add some HCl to this aqueous solution.  If nothing happens, then the compound is ZnCl2.  If some brown nitrogen oxides are given off, then you have the aluminum nitrate.  

So use the HCl.   ;D
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Offline AWK

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Re:Qualitative Tests
« Reply #7 on: December 16, 2004, 02:10:04 AM »
Quote
Indeed, but AWK posted various tests that work with the use of various reagents that cannot be formed from reactions between the powders we get or our one reagent and a powder
Chemist almost always use solution or water suspention. this is possible because deionized water is mensioned in equipment part.

Quote
2):  Na2SO3 and HCl will give off an odor of rotted eggs which will be fairly noticeable.
This is odor of H2S, not SO2; none the less the odor of SO2 is specific.

Quote
KI + HCl will result in the formation of HI vapor which will be very noticeable as the solution turns a red orange color.
HCl is more volatile than HI - what about Law of Mass Action
But sulfite and iodide react in acidic solution to produce iodine

There is no possibility to identify all this substances using only one reagent.
But idea of Jdurg is nice - using of identified substance from the list is not forbidden
« Last Edit: December 16, 2004, 02:40:41 AM by AWK »
AWK

Offline jdurg

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Re:Qualitative Tests
« Reply #8 on: December 16, 2004, 11:31:18 AM »
Chemist almost always use solution or water suspention. this is possible because deionized water is mensioned in equipment part.This is odor of H2S, not SO2; none the less the odor of SO2 is specific.HCl is more volatile than HI - what about Law of Mass Action
But sulfite and iodide react in acidic solution to produce iodine

There is no possibility to identify all this substances using only one reagent.
But idea of Jdurg is nice - using of identified substance from the list is not forbidden

I did not know that the sulfite ion and the iodide ion would react to produce iodine.  That's nice and easy to see.  :-)  Also, while the odor of SO2 isn't that of rotten eggs, it smells close enough.   :P
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Offline AWK

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Re:Qualitative Tests
« Reply #9 on: December 17, 2004, 01:49:49 AM »
Quote
But sulfite and iodide react in acidic solution to produce iodine

My information is rather confused. Iodine, not iodide reacts with sulfite

And the problem is still unresolved!
« Last Edit: December 17, 2004, 01:51:07 AM by AWK »
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Offline jdurg

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Re:Qualitative Tests
« Reply #10 on: December 17, 2004, 02:37:49 PM »
Still, I think that if you added some HCl to the raw KI powder you would see something happen.  (Perhaps someone would be willing to try this out and test it?)   ;)
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budullewraagh

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Re:Qualitative Tests
« Reply #11 on: December 17, 2004, 03:22:30 PM »
i agree that there would be a displacement reaction between KI and HCl yielding HI and KCl, considering the activity series and the fact that the reaction would occur under aqueous conditions

Offline Donaldson Tan

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Re:Qualitative Tests
« Reply #12 on: December 26, 2004, 06:40:53 PM »
you could work out a grid of possibilities and make a grid of your experimental results. then compare them. it's an analytical method that should help you arrive at your answer.
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