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Topic: Determine an Unknown Salt in a lab  (Read 18489 times)

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blahblah

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Determine an Unknown Salt in a lab
« on: May 16, 2005, 10:56:57 PM »
I need to know, and have tried to figure out:
1. percentage weight loss on heating the solid - i tried doing this by just heating the salt in a glass... it melted... then when it cooled I weighed it and it weighed more than the starting point, which was weird, so i probably did something wrong.
2. solubility of the salt in water - do I use a spec-20 for this?
3. melting point - i already measured this with this weird melting machine, it was about 305 degrees
4. positions of the brightest lines in the emission spectrum - or is /this/ what you use a spec-20 for?
5. identification of cation and anion by solubility precipitation reactions - i suspect that mine is a double salt, with more than one cation or more than 1 anion because the melting point is so high, but i dont know how to figure out any more than that

how do i do these with limited lab equipment?
« Last Edit: May 16, 2005, 10:58:13 PM by blahblah »

Froggirl

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Re:Determine an Unknown Salt in a lab
« Reply #1 on: May 17, 2005, 02:05:55 AM »
1. don't always think on heating/burning that your compounds should loose mass. you might be forming some sort of oxide or something?? My suggestion is that you go back and redo this bit and see if you get the same thing.

2. your solubility can be worked out by simply dissolving small amounts of your salt until no more can be dissolved!! just keep track of each addition you make, and make sure that you keep your solution at about 20oC so it matches standard lab conditions which most references will be at. final result will be in grams per litre (g/L)

4. Spectrum/spec-20 see the connection? I assume this is just your run of the mill spectrophotometer and yes this will be what gives you the absorption wavelengths for you compound. This will just be a matter of dissolving your compound in the right solvent and running a spectrum with the machine and just picking out your major peaks.

5. Ok well this is where solubility rules come in. most first year chem textbooks will have a list of them but this website is also a good source:

http://www3.baylor.edu/~Kenneth_Busch/Solubility_Rules.html

Basically you have to go through a process of adding different solutions to a solution of your unknown and see if aything precipitates out. Each time something precipitates out it gives you a clue as to what it is.
Also have a look at this procedure:

http://www.uncp.edu/home/mcclurem/courses/chm226/Salt_Unknown.pdf

Looks like a fair bit like what you've been asked to do.   :)

Garneck

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Re:Determine an Unknown Salt in a lab
« Reply #2 on: May 17, 2005, 09:55:46 AM »
Good instruction, froggirl. Although there could be a few other possible cations in that specific salt.

Froggirl

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Re:Determine an Unknown Salt in a lab
« Reply #3 on: May 18, 2005, 08:01:21 PM »
phew...because I'm actually an organic chemist  :)

Garneck

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Re:Determine an Unknown Salt in a lab
« Reply #4 on: May 19, 2005, 01:52:05 AM »
phew...because I'm actually an organic chemist  :)

that explains probably just about everything ;)

Offline Donaldson Tan

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Re:Determine an Unknown Salt in a lab
« Reply #5 on: May 20, 2005, 12:28:51 AM »
garneck: what chemist are you then? organic?
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Garneck

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Re:Determine an Unknown Salt in a lab
« Reply #6 on: May 20, 2005, 07:09:47 AM »
garneck: what chemist are you then? organic?

no, analytical

Offline Borek

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Re:Determine an Unknown Salt in a lab
« Reply #7 on: May 20, 2005, 10:36:18 AM »
Which lab?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Garneck

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Re:Determine an Unknown Salt in a lab
« Reply #8 on: May 20, 2005, 01:24:09 PM »
Which lab?

Lab. of Applied Analytical Chemistry, prof. Golimowski

blahblah

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Re:Determine an Unknown Salt in a lab
« Reply #9 on: May 26, 2005, 01:21:44 AM »
Thanks for all the *delete me*

Hmm.. we figured out that part of it is sodium. But sodium is soluble in, well, e verything, so how do I figure out the other ion? I suspect it is sodium nitrate, but I don't know how to tell.

Offline xiankai

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Re:Determine an Unknown Salt in a lab
« Reply #10 on: May 26, 2005, 07:35:49 AM »
nitrates , when added with sodium hydroxide and a bit of aluminium powder(catalyst), produce ammonia, a strong smelling gas when heated.
one learns best by teaching

blahblah

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Re:Determine an Unknown Salt in a lab
« Reply #11 on: May 28, 2005, 01:17:13 AM »
Ahh, we couldn't get ahold of aluminum powder... (I know my chem instructor has some tucked away, but he probably thought we would blow ourselves up). Both sodium and nitrate are soluble in everything, so it's really hard to tell if it's that or something else based on the charts. We've just been working by the process of elimination, but we still can't find something that will indicate that nitrate is in it. We are pretty sure it is in fact sodium nitrate, just by looking at pictures and having the same melting point, but we just don't know how to prove it for sure.

asqa

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welcome
« Reply #12 on: June 16, 2005, 07:49:34 AM »
hai,

welcome to chemical forum
thanks 8)

Alkrav

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Re:Determine an Unknown Salt in a lab
« Reply #13 on: June 17, 2005, 02:05:58 PM »
What about flame tests? Sodium has a caracteristic yellow flame! Other cations have also caracteristic colors!

Offline xiankai

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Re:Determine an Unknown Salt in a lab
« Reply #14 on: June 17, 2005, 10:50:29 PM »
What about flame tests? Sodium has a caracteristic yellow flame! Other cations have also caracteristic colors!

not sodium in solution i think
one learns best by teaching

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