Sciencemadness Discussion Board - Identifying Unknown Compounds in a Lab

Identifying Unknown Compounds in a Lab

a.dunn.wa - 31-5-2012 at 15:42

Hello,

I am looking for assistance from you chemistry maestros, geniuses, and experts on how to handle a 5-day lab of identifying unknown compounds using limited resources.

We were given four unknown compounds and a list of fifty possibilities. I am not as interested as in you guys helping me identify them (atleast not yet), but more interested on how you guys would tackle this assignment--no procedure is given to us, we can do whatever we want given the resources we are provided with.

Resources given are as follows:
Water
Ethanol
Red and blue litmus paper
0.1 M silver nitrate solution*
Bromothymol blue indicator
Phenolphtalein indicator
0.1 M NaOH solution
0.1 M K₂SO₄ solution*
phenol red indicator
0.1 M HCl
0.1 M H₂SO₄

And then stuff like hot plates, B.B.s, elec. conductivity meters, thermometers etc. are also provided.

So basically I have two questions:
1.) What would your rough procedure look like and why (what each test would accomplish)

and

2.) What do all of these given testing chemicals do? I know what some do but the ones with asterisks I put next to I am not really sure of, and I hope you can help me out.

I appreciate your guy's help so much, and I hope you take the time to reply if you can help. It means a lot to me!

Andy

Vargouille - 31-5-2012 at 16:18

http://cms.imperial.edu/index.php?pid=1205

Check the solubility rules.

Sublimatus - 31-5-2012 at 17:57

This flowchart will take you pretty far:
http://i.imgur.com/HdQMN.png

Most of the instructions are at the bottom of the page. Just remember to use a new (small) portion of your unknown for each step in the flowchart.

Also, I meant to mention this page is from Introduction to Organic Laboratory Techniques A Microscale Approach, 4th Ed. by Pavia, Lampman, Kriz, and Engel.

[Edited on 6/1/2012 by Sublimatus]

Mailinmypocket - 31-5-2012 at 18:16

What are the first steps you are planning on trying or what have you already tried? Have you tried testing any of the mystery chemicals with the silver nitrate and potassium sulfate to see what happens?

[Edited on 1-6-2012 by Mailinmypocket]

Mixell - 31-5-2012 at 18:20

Water and ethanol are mostly to separate chemicals by solubility.

litmus papers are for checking if the solution is acidic or basic.

Silver cations create insoluble silver halides of the formula AgX (X=Cl/Br/I).

bromothymol, phenolphtalein phenol red are indicators for acid/base when doing titrations (with HCl/H2SO4 or NaOH).

K2SO4 is an indicator to the presence of group 2 metal cations starting from Calcium, which create the insoluble salts of the formula MSO4 (M= Ca/Sr/Ba).

thermometers are for checking the enthalpy of dissolution of various salts/melting and boiling points and so on.

conductivity meters are useful if you want to determinate if a solution is conductive, if it is, its probably a solution of salt.

Sorry for the lousy format, its 5 AM here, I couldn't bother...

[Edited on 1-6-2012 by Mixell]

a.dunn.wa - 1-6-2012 at 14:32

UPDATE:

Found some interesting new info on one of the unknowns through a simple solubility in water test. This compound turned yellow when dissolved with water, although it takes some agitating to get it to dissolve. Also, when initially poured in, some of the compound floated while some sank which gave us the impression that it's density is very close to 1. If you think you know what this could mean, or if you want to take a stab at guessing the compound, we would really appreciate your input!

Here's a picture and the list of possibilities:

POSSIBILITIES:
Aluminum hydroxide
Aluminum nitrate
Ammonium chloride
Barium sulfate
Calcium chloride
Calcium nitrate
Calcium sulfate
Iron II sulfate
Iron III sulfate
Lauric acid
Magnesium carbonate
Magnesium hydroxide
Magnesium sulfate
Potassium bromate
Potassium carbonate
Potassium chloride
Potassium iodide
Potassium metabisulfite
Potassium nitrate
Potassium perulfate
Potassium phosphate
Potassium sodium tartrate
Potassium thinocyanate
Sodium acetate
Sodium bisulfate
Sodium borate
Sodium bromide
Sodium carbonate
Sodium chloride
Sodium citrate
Sodium fluoride
Sodium iodide
Sodium nitrate
Sodium nitrite
Sodium oxalate
Sodium phosphate
Sodium sulfate
Sodium sulfite
Sodium thiosulfate
Starch
Stearic acid
Strontium chloride
Strontium hydroxide
Strontium nitrate
Sucrose
Tin II chloride
Urea
Zinc nitrate
Zinc sulfate

a.dunn.wa - 1-6-2012 at 14:35

Quote: Originally posted by Mailinmypocket

What are the first steps you are planning on trying or what have you already tried? Have you tried testing any of the mystery chemicals with the silver nitrate and potassium sulfate to see what happens?

[Edited on 1-6-2012 by Mailinmypocket]

Not yet we are planning on doing that Monday/Tuesday. Today (Friday) was our first day being able to play with the unknowns.

White Yeti - 1-6-2012 at 14:39

Strange, they don't give you any sodium bicarbonate. Sodium bicarbonate can be useful in identifying certain substances.

Amm - 1-6-2012 at 15:57

Here is a picture of the compound that made the yellow solution before it was dissolved in water.

Amm - 1-6-2012 at 16:08

Photo:

Vargouille - 1-6-2012 at 16:18

Just wiki all of the compounds for solubility and colors, both in solid and in solution.

barley81 - 1-6-2012 at 16:34

You cannot measure the density that way. First, the solid may have floated due to surface tension. Second, air is incorporated into the powder, so the density of the solid is not the same as the density of the solid.

This is kind of puzzling because none of the compounds would normally make a yellow solution. It could be anhydrous (or weakly hydrated) iron II sulfate, which is white-ish, and forms a brownish/yellowish solution due to oxidation to iron III...

Mixell - 1-6-2012 at 16:54

I suggest ignoring the floating part if its a fine powder. Fine powders of materials that do not dissolve readily in water (require agitation at minimum) tend to float because of surface tension of water. For example very fine nickel flakes float on the surface of water like a charm, I literally couldn't get them to sink.

Now, all of the first and second (tin and zinc too) group metal cations are colorless in solution, so you can rule them out (the anions you specified are also colorless).
Iron II is light green in color, and iron III is yellow (a good candidate).

All of the other chemicals either do not dissolve in water, or are colorless (not so sure about urea though, heard it can be yellowish).

Anyway, check the pH, if its acidic, its most definitely Iron III.
Too bad you don't have some barium nitrate to check for the sulfate ion to make a 100% determination.

[Edited on 2-6-2012 by Mixell]

Mixell - 1-6-2012 at 17:03

Barley,
I think quite a lot of oxidation is required for the color of the solution to shift to this shade of yellow, it would not happen immediately using distilled water.
Also, according to a Wikipedia picture, Fe2(SO4)3 can have a pretty off-white color (especially if the photographing conditions are shite): http://en.wikipedia.org/wiki/File:S%C3%ADran_%C5%BEelezit%C3...

Anyway, make sure that the solution is fresh, the water is distilled and check the pH!

barley81 - 1-6-2012 at 18:52

Iron II is oxidized fairly rapidly in distilled water (neutral solution) and dissolved oxygen can certainly produce a yellow color from iron III (hydrolyzed species) in this solution. Plus, the solid may have oxidized in the air, producing more iron (III) compounds. You're right that it could be ferric sulfate.

Both ferrous sulfate and ferric sulfates are not yellow! Does this mean that basic iron III compounds are present in the solid? I have some ferrous sulfate heptahydrate that I left out for a short while, and it has become tinged yellow/brown even though some excess sulfuric acid was trapped in the crystals.

Mixell - 2-6-2012 at 01:30

Define "fairy rapidly" please, because I've dissolved ferrous sulfate, and it didn't develop any noticeable yellow tinge (certainly not the one shown in the picture) during the several minutes when the solution was not in use.

And according to Wikipedia and google pictures, ferric sulfate can be off-white to yellow.

May be the topic starter can clarify whether the yellow color appeared right away or after some time. And the exact color of the solid compound.

[Edited on 2-6-2012 by Mixell]

barley81 - 2-6-2012 at 03:39

If you leave a sample of solid ferrous sulfate out, it becomes brown-colored in a while (hours/minutes ?).

Quote:

If for some reason you need ferrous sulfate with very low acid content, you'll have to take your chances with huge losses to oxidation by trying to grow the crystals at a higher pH (near neutral). Such a solution can be made by adding slightly more steel wool to the acid than it can theoretically dissolve. It may then become necessary to do the whole experiment under an inert atmosphere (nitrogen or argon). There will probably still be traces of H2SO4 in the crystals despite this extra effort.

Fairly rapidly = fast enough that crystal-growing becomes difficult due to formation of rusty-crap.
http://www.crscientific.com/ferroussulfate.html

[Edited on 2-6-2012 by barley81]

Mixell - 2-6-2012 at 05:07

I think hours is the more accurate time frame.
And I don't think he messed with the solution for hours before taking a photo (or dropped it into a hydrogen peroxide solution).

AJKOER - 2-6-2012 at 07:35

First, closely examine the structure of the salts. Color, of course, and smell.

You may also be able to determine the precise solubility in water. Also, using H2SO4 (or NaOH ), precisely how basic (or acidic) the salt is using a litmus test.

Heat the salt and record the temperature at which it starts to melt (or boil) and/or decompose. Capture some fumes and dissolve in water and note smell. If carbon remains, you have an organic compound. You may be able to determining density of melted salt.

Heat with HCl, observe any gas colors and also carefully note smells again. For example, nitrates will produce fumes of Nitric acid, sulfites SO2, acetates smell of vinegar, citrates smell of citric acid, bromides HBr (caution), iodides I2 vapors,...

If no reaction with HCl, do NOT react with H2SO4 as you may have NaF and heating with H2SO4 can produce deadly HF fumes.

Do a flame test to isolate Na, K, Zn, Fe, Ba, Mg, Sn and Sr.

[Edited on 2-6-2012 by AJKOER]

a.dunn.wa - 2-6-2012 at 08:20

Quote: Originally posted by Mixell

Define "fairy rapidly" please, because I've dissolved ferrous sulfate, and it didn't develop any noticeable yellow tinge (certainly not the one shown in the picture) during the several minutes when the solution was not in use.

And according to Wikipedia and google pictures, ferric sulfate can be off-white to yellow.

May be the topic starter can clarify whether the yellow color appeared right away or after some time. And the exact color of the solid compound.

[Edited on 2-6-2012 by Mixell]

The yellow color did not appear immediately, it appeared after we agitated the solution for maybe 5-10 seconds. We ran the test 2x with this unknown, due to it turning yellow we wanted to observe it again, and it does turn a shade of yellow once some of the compound starts to be dissolved; and once it is all dissolved it reaches the shade photographed.

The solid compound's color is a yellowish-whitish color. We also noted that it''s initial appearance is granular. I will try to get a better picture with decent quality on Monday of the compound again.

Thanks everyone for the input! It is really helpful

Mixell - 2-6-2012 at 11:46

Well here you go, yellow directly on dissolution, yellowish as a solid.
Just check the pH, and may be with barium nitrate to be 100% sure, and if the results are positive (acidic solution and sulfate present), it is Fe2(SO4)3 (Fe III sulfate).

Amm - 4-6-2012 at 09:00

Hi I'm the lab partner for this experiment, we don't have barium nitrate any other suggestions

Vargouille - 4-6-2012 at 09:11

Strontium nitrate also does it, but it's not quite as good, because SrSO4 is more soluble in water than BaSO4. Even worse is calcium chloride or calcium nitrate, but if you really have no soluble strontium salt, it could work.

If you don't have any of those, there might be some way of identifying a sulfate that I'm forgetting.

Amm - 4-6-2012 at 09:25

We have k2so4 and h2so4, I will post photos of the outcomes of mixing each thing with those when I have access to a computer

Hexavalent - 4-6-2012 at 09:58

The guys doing the experiment . . .do you have to carry out your work in the fumehood?

Mixell - 4-6-2012 at 10:14

Nah, mixing with K2SO4 or H2SO4 will do nothing if its Fe III sulfate.
Just check the pH of the solution!

barley81 - 4-6-2012 at 10:45

Add NaOH and you should get rusty brown precipitate indicating iron III.

Since you have identified iron III, it can only be the sulfate because that's the only iron III salt possible (your list).

Mixell - yeah, it's ferric sulfate. Sorry 'bout that. If you look up pictures on google images, many samples are yellow as well...

[Edited on 4-6-2012 by barley81]

Amm - 4-6-2012 at 10:48

No we don't have to work under the fume hood, we did mix it with NaOH, and it made a rusty solution

Mixell - 4-6-2012 at 11:16

So I think we made a conclusion =)
Next compound?

Hexavalent - 4-6-2012 at 11:16

Do you mean a rusty-coloured solution/precipitate?

I second what barley said....iron (II) and iron (III) compounds tend to have brownish hydroxides that are insoluble IIRC.

Next compound?

[Edited on 4-6-2012 by Hexavalent]

Amm - 4-6-2012 at 11:51

Ok we have 3 other unknown subsatances, any knowledge you can share is greatly appreciated.

Unknown letter A- nothing note worthy happened when mixed with NaOH, K2SO4 or AgSO4
Appeared to dissolve in each

Unknown letter B
-Mixed with AgSO4

It formed white soft blobs that stuck to the side of the glass

Unknown letter c
-mixed with NaoH

Turned a white, milky color

-Mixed with K2SO4

made a cloudy solution appeared to be a precipitate

-Mixed with AgSO4

Turned brown or purple

Unknown letter D (suspected Iron III Sulfate)
- Turned each solution a rusty orange color

a.dunn.wa - 4-6-2012 at 12:47

Quote: Originally posted by Hexavalent

The guys doing the experiment . . .do you have to carry out your work in the fumehood?

Well we do have access to just simple lab stations (just tables) as well. But I'm intrigued to see where you are going because, if it is something that would be really helpful I think we could convince our teacher to let us perform it.

a.dunn.wa - 4-6-2012 at 12:50

Quote: Originally posted by Mixell

Nah, mixing with K2SO4 or H2SO4 will do nothing if its Fe III sulfate.
Just check the pH of the solution!

How would one go about checking the pH. Just want to make sure we are on the same boat here, because this is what we're planning on doing:
Adding water to the unknown compound; then using a pH strip to test it. Is that all there is to it?

Also how do you guys know this is Fe III and not Fe II sulfate? Would the pH reveal that? What is the pH of these two compounds supposed to be?

Sorry about all the questions, we really appreciate your guys time!!!

Mixell - 4-6-2012 at 12:55

Fe II solution should be neutral, Fe III should be acidic:
Fe3+ +H2O --> Fe(OH)2+ + H+
Use the blue litmus paper, if its acidic, it will turn red.

Mixell - 4-6-2012 at 13:33

Remember: always add the chemicals gradually with mixing.

I'll start from C:
Strontium and calcium hydroxide (especially the calcium hydroxide) are not very soluble.
Also the sulfates are pretty insoluble.
But the brown/purple suspension makes me think that its a suspension of silver, thus there is a reducing agent in the solution.
Tin II (and Fe II) is a reducing agent, but should not form a precipitate with sulfate.
And the only somewhat reducing sugar on your list is starch, but it is not soluble in water.

Please check the conductivity of the solution.

And if you can, add some nitric acid to your solution and heat it up (do it in the fume hood) and report if a brown gas appears (NO2) or add sulfuric acid and hydrogen peroxide. Then check the solution with silver nitrate (or sulfate if there is no nitrate and report if there is a precipitate (it should be a very noticeable precipitate).

B:
I assume it only reacts with the silver sulfate?
Then it is a solution of chloride/bromide/iodide.
Add some nitric acid to it, or hydrogen peroxide and sulfuric acid. If the color of the solution turns purple and there is a black/purple precipitate, then its iodine. If it turns yellow (noticeably) it is bromine. And finally, add sulfuric acid +hydrogen peroxide/ KMnO4 and put a moist KI/NaI (iodine paper) on top of the test tube, may be even try to seal it with your thumb for a faster result. If the iodine paper turns purple, then its chlorine.
And to detect if its sodium or potassium, use a flame test.

A:
Check the conductivity of the solution and report.

Amm - 5-6-2012 at 10:50

Unknown letter c
- We burned this unknown, it appeared to produce a liquid
-burned a white color

During burning

After burning

-We also Mixed it with our Unknown letter D, identified as iron III sulfate
- Appeared that a precipitate formed

a.dunn.wa - 5-6-2012 at 11:00

UPDATE #2:

Alright, so here is where we are at guys. We have two days left in the lab (on Thurs and Fri) and that's it to identify these three unknowns.
My lab partner has the pics for what they are worth, and will be posting them later.

Unknown D (suspected Fe): Tested pH of it today w/ blue litmus paper. Turned the paper red. As stated by Mixell Fe III should be acidic so we are going to conlclude that unknown D is Fe III sulfate.

Unknown C: Still not sure what it is, but we have a lot of results for it.
---C + water --> cloudy white precipitate

---Using phenol red, we found C to be basic

---Using distilled water, we found the elec. conductivity of C to be medium...the equipment we used just had a green and red light and a general key on the back.

---C + a sulfate (either H2 or K2, drawing a blank right now) --> a precipitate

---When C was burned it bubbled and turned brown over time...is this hydrolysis? (we do have a pic of this)...I know tin II chloride would hydrolysize...what do you guys make of this??

---burned white color

---In 0.1 M NaOH, C created a white, milky precipitate

Unknown B:

---Using phenol red, B was determined to be basic

---Elec conductivity in distilled water was high

---burned bright, vivid, persistent, orange

Unknown A:

---highelec. conductivity in distilled water

---burned deep, strong orange color

So what do you guys make of all this? We really appreciate any help/assistance you guys can give us...also are there any more tests you would have us try? The chemicals we have available are as follows (same as in OP):
Water
Ethanol
Red and blue litmus paper
0.1 M silver nitrate solution*
Bromothymol blue indicator
Phenolphtalein indicator
0.1 M NaOH solution
0.1 M K2SO4 solution*
phenol red indicator
0.1 M HCl
0.1 M H2SO4

Thanks guys!

Hexavalent - 5-6-2012 at 11:02

a.dunn.wa - AJKOER said 'If no reaction with HCl, do NOT react with H2SO4 as you may have NaF and heating with H2SO4 can produce deadly HF fumes.'

I'm surprised that they gave you NaF in the list of possibilities, particularly if you don't work in the fumehood. Although the amounts would be very small, HF gas works wonders to the human body in the most horrific of ways.

Most of the other tests are fairly nonhazardous, however, and I don't think they *would* give you NaF if you also had H2SO4.

a.dunn.wa - 5-6-2012 at 12:31

Also worth noting is that Unknown Substance B is hygroscopic. Which limits it down to these according to Wikipedia:

1. Aluminum nitrate
2. Ammonium chloride
3. Calcium chloride
4. Calcium nitrate
5. Magnesium carbonate
6. Magnesium sulfate
7. Potassium carbonate
8. Sodium bisulfate
9. Sodium carbonate
10.Sodium iodide
11. Sodium nitrite
12. Sodium sulfate

The fact that it burns orange makes me believe it has calcium in it. What do you guys think?

Hexavalent - 5-6-2012 at 13:08

No, calcium's flame test is brick red. Orange points to a sodium compound, usually, but the orange could be masking another, maybe weaker colour, such as the white exhibited by magnesium compounds, the very pale green[1] seen by ammonium species etc. Do you have access to a cobalt glass square or something to see the proper colours of the flame?

From there, you can check pH for sodium bisulfate, reaction with an acid for the carbonate, AgNO3 precipitation test for the iodide, heating it up and see if it decomposes if its the nitrite (and what products are formed). The sulfate could then either be determined by the process of elimination, OR by taking a melting point (884 °C (anhydrous)
32.38 °C (decahydrate)) or begging your teacher for some barium chloride solution

You could also possibly look up some melting points for the otherchemicals, and determine which compound you have that way....although you may have trouble as many ionic compounds have really high melting points.

[1] From 'The Illustrated Guide to Home Chemistry Experiments', Laboratory 19.1 - Use Flame Tests to Discriminate Metal Ions

[Edited on 5-6-2012 by Hexavalent]

Mixell - 5-6-2012 at 19:20

C:
A good chance it is Tin II chloride, please check with the procedure I described.

B:
Might be sodium carbonate or sodium acetate/borate/oxalate/citrate. Add H2SO4, if bubbles appear, it's the carbonate. If it smells like vinegar, it's the acetate. Report.

A:
Might be sodium fluoride/nitrate/sulfate/ etc.. check the pH please.

Amm - 6-6-2012 at 08:05

Could we identify C using a burn test? we don't have access to a lot of chemicals.
Also Unknown A is basic.

Mixell - 6-6-2012 at 08:23

A:
If A is basic, then it's probably the salt of sodium and a weak acid, but a one that does not form a precipitate with silver ions.
It might be citrate/borate.

B:

Do the acid test, and report if bubbles are formed (and/or any other effects or smells, like the smell of vinegar).

C:

Test with silver nitrate and report (very likely it is Tin II chloride).

Amm - 7-6-2012 at 09:11

We mixed b with h2so4 and it smelled like vinager we are thinking sodium acetate

We mixed c with silver nitrate and it formed a precipitate we are thinking tin ii chloride

[Edited on 7-6-2012 by Amm]

[Edited on 7-6-2012 by Amm]

Hexavalent - 7-6-2012 at 09:52

Those are possible, just please, when writing formula, be more careful with your lettering, e.g. sulfuric acid is H₂SO₄, not h2so4. At the least, use H2SO4, but the use of subscript as seen here is favourable.

We understand what you mean, but later in your chemistry career when things get more difficult, it will be absolutely necessary - e.g. Co is cobalt, but CO is carbon monoxide . . .two very different things.

[Edited on 7-6-2012 by Hexavalent]

a.dunn.wa - 7-6-2012 at 11:45

UPDATE:

Final update. Tomorrow we have 50 mins, and then our identifications of the compounds are due. We still need help identifying three, but we feel confident that we have enough data and info to nail these. Please help any way you guys can.

1. Our teacher gave us help by letting us check our answers once with the correct answers, and gave us some helpful hints while doing so.

For Unknown A: we guessed sodium sulfate. He said that it does have Sodium, but is not sulfate. Our new hypothesis is that is is sodium acetate, because it has a distinct, vinegar-like odor, when mixed with HCl. Other pieces of supporting evidence are that the compound is ACIDIC, burns yellow/orange (sodium indication), and forms a white precipitate with AgNO₃

For Unknown B: we gussed sodium acetate. He said that it does have sodium, but it is not acetate. We are not sure what B is. Other observations about B is that it is very hygroscopic. When mixed with H₂SO₄ a strong, pungent, vinegar smell is made (this made us think acetate...). Other tests showed that it dissolved, colorless in NaOH, burned a strong orange color (indication of sodium), was found to be basic, and had a high elec. conductivity.

For Unknown C: we gussed potassium bromate. He said that it doesn't have either, but the second part is very close to bromate. He eventually told us it was either Cl or I for the second part. The main point of evidence is that when burned it burned a white/purple color, which we took as potassium but he said that another metal on our list burns purple/white. Does anyone know what that metal is??? Other facts about this compound include that it made a white, milky precipitate when mixed with NaOH, made a purple-colored solution when mixed with AgSO₃, is basic, and makes water a cloudy white color when mixed. Anyone have an idea??

We really appreciate all your guys help. It has been beyond helpful for us. If you need to know anything else, please let us know, and we will try to help you guys help us.

a.dunn.wa - 7-6-2012 at 11:51

Also, just for reference again, here are the list of all the possibilities:

Aluminum hydroxide
Aluminum nitrate
Ammonium chloride
Barium sulfate
Calcium chloride
Calcium sulfate
Iron II sulfate
Iron III sulfate
Lauric acid
Magnesium carbonate
Magnesium hydroxide
Magnesium sulfate
Potassium bromate
Potassium carbonate
Potassium chloride
Potassium iodide
Potassium metabisulfite
Potassium nitrate
Potassium persulfate
Potassium phosphate
Potassium sodium tartrate
Potassium thiocyanate
Sodium acetate
Sodium bisulfate
Sodium borate
Sodium bromide
Sodium carbonate
Sodium chloride
Sodium citrate
Sodium fluoride
Sodium iodide
Sodium nitrate
Sodium nitrite
Sodium oxalate
Sodium phosphate
Sodium sulfate
Sodium sulfite
Sodium thiosulfate
Starch
Stearic acid
Strontium chloride
Strontium hydroxide
Strontium nitrate
Sucrose
Tin II chloride
Urea
Zinc nitrate
Zinc sulfate

Amm - 7-6-2012 at 12:18

[Edited on 7-6-2012 by Amm]

a.dunn.wa - 7-6-2012 at 12:57

After thinking about this for another hour now, I think we can draw a couple more conclusions...
Unknown C is Tin II sulfate. Since the compound burns a purple/white color and is not potassium, a lot of possibilities are eliminated (all potassium, sodium, strontium, aluminum, calcium, iron, barium compounds). Then once all of those are eliminated, I took into consideration the fact that it HAS to have chlorine or iodine. That leaves tin II chloride as really the only logical choice left. And we also got something very very similar as shown in this video someone else took (assuming they started with water):http://www.youtube.com/watch?v=epd-DAE_sE When they add in the tin ii sulfate that is pretty much what our unknown did. Please let me know if you agree/disagree, or if there is any problems in my reasoning that aren't technically sound.

Unknown A is sodium acetate. First, I started with the definites. Our teacher told us that this had sodium guaranteed, so everything without sodium was eliminated. That narrows it down to 16 minus the one sodium we already guessed (sulfate). The fact that the compound is acidic (sodium acetate is acidic as I found here http://answers.yahoo.com/question/index?qid=20110525035727AA...) and the fact that it had a distinct vinegar odor makes me believe it is sodium acetate.

Unknown B I am still not sure of. Since it has sodium, and isn't sodium acetate, that narrows it down to 15 possibilities. The fact that it is basic eliminates some possibilties but I'm still not sure...anyone have any ideas for this one? Maybe citrate?

Hexavalent - 7-6-2012 at 13:08

I believe sodium acetate is actually basic . . .the acetate ion acts as a Bronstead-Lowrey base in a reaction with water, or, in other terms, sodium acetate dissociates in water into sodium and acetate ions. Sodium ions react very little with hydroxyl ions whereas acetate ions combine with hydrogen ions to produce neutral acetic acid, and the net result is a relative excess of hydroxyl ions, causing a basic solution.

Here the basic properties of sodium acetate are taken advantage of; http://www.youtube.com/watch?v=kH19EIf5GtE
and here;

http://www.youtube.com/watch?feature=iv&annotation_id=an...

a.dunn.wa - 7-6-2012 at 13:47

Quote: Originally posted by Hexavalent

I believe sodium acetate is actually basic . . .the acetate ion acts as a Bronstead-Lowrey base in a reaction with water, or, in other terms, sodium acetate dissociates in water into sodium and acetate ions. Sodium ions react very little with hydroxyl ions whereas acetate ions combine with hydrogen ions to produce neutral acetic acid, and the net result is a relative excess of hydroxyl ions, causing a basic solution.

Here the basic properties of sodium acetate are taken advantage of; http://www.youtube.com/watch?v=kH19EIf5GtE
and here;

http://www.youtube.com/watch?feature=iv&annotation_id=an...

That first video is pretty neat. Do you still think it could be/is sodium acetate? I feel like since HCl + Unknown --> a strong, vinegar smell it must be the acetate..I don't know any others that would do that

Hexavalent - 7-6-2012 at 13:55

I think the first sentence on the link you provided is a typo.

The best bet would be to try and test the pH again in your lab, perhaps there was just something wrong with the indicator sample you used? Or the reaction vessel? Or maybe you even used the wrong sample to begin with . . .even scientists make mistakes sometimes

There is a good chance that it is sodium acetate.

By adding HCl, you would get acetic acid and sodium chloride. This reaction is favored because of the formation of a weaker acid.

NaC2H3O2 + HCl --> HC2H3O2 + NaCl

If it's a net ionic equation you want, just eliminate the spectator ions, which in this case are Na+ and Cl-, the only ions that remain in solution:

H+ + C2H3O2- --> HC2H3O2

Do you think it's sodium acetate trihydrate or anhydrous? If its the trihydrate, add some to a test tube and test the MP by heating very slowly with a Bunsen burner, maybe even in a water bath if you can. The trihydrate melts at 58*C, which is surprisingly low for an ionic solid.

[Edited on 7-6-2012 by Hexavalent]

Mixell - 7-6-2012 at 13:58

C:
Probably Sn II chloride, as the teacher said it has chloride (or iodide) ions in it, and it's a reducing agent.
A:
If it bubbles on contact with acid, it is sodium carbonate. Sodium acetate is not acidic, it is basic.

Also, you contradict yourself:

"Unknown letter A- nothing note worthy happened when mixed with NaOH, K2SO4 or AgSO4
Appeared to dissolve in each"

"For Unknown A: we guessed sodium sulfate. He said that it does have Sodium, but is not sulfate. Our new hypothesis is that is is sodium acetate, because it has a distinct, vinegar-like odor, when mixed with HCl. Other pieces of supporting evidence are that the compound is ACIDIC, burns yellow/orange (sodium indication), and forms a white precipitate with AgNO3"

Sodium nitrate is fairly soluble, so it will not form a precipitate, so did the solution form a precipitate with silver ions or not?

You don't have any acidic sodium salt (unless the sodium phosphate is NaH2PO4) that will form a precipitate with silver. The only acidic salt of sodium you have is the bisulfate.

B:
The basic (and not reducing) sodium salts you have are: phosphate, borate, citrate, oxalate, carbonate (not including acetate).
Test it with acid, if bubbles are formed, it is the carbonate.

a.dunn.wa - 7-6-2012 at 15:17

Quote: Originally posted by Hexavalent

I think the first sentence on the link you provided is a typo.

The best bet would be to try and test the pH again in your lab, perhaps there was just something wrong with the indicator sample you used? Or the reaction vessel? Or maybe you even used the wrong sample to begin with . . .even scientists make mistakes sometimes

Your right about the typo, sorry about that. Here's the link I intended to paste:
http://www.youtube.com/watch?v=epd-DAE_sEI
This is sort of as much for my partner to see as well, because this looks exactly like ours...

We will definitely test the pH again tomorrow. Chances are something just went wrong or we made a mistake, happens a lot to us in chemistry

How would one go about determining/deciding if a compound is anhydrous or trihydrate? I would suspect it to be somewhat hydrated because we have been opening the container quite a bit and I'm sure some of the water from the air was captured/sucked up by the compound (espec. since its hygroscopic).

Either way, we may just try to find the melting point as we have 50 minutes, and we may not have that many other tests to run.

a.dunn.wa - 7-6-2012 at 15:23

Quote: Originally posted by Mixell

C:
Probably Sn II chloride, as the teacher said it has chloride (or iodide) ions in it, and it's a reducing agent.
A:
If it bubbles on contact with acid, it is sodium carbonate. Sodium acetate is not acidic, it is basic.

Also, you contradict yourself:

"Unknown letter A- nothing note worthy happened when mixed with NaOH, K2SO4 or AgSO4
Appeared to dissolve in each"

"For Unknown A: we guessed sodium sulfate. He said that it does have Sodium, but is not sulfate. Our new hypothesis is that is is sodium acetate, because it has a distinct, vinegar-like odor, when mixed with HCl. Other pieces of supporting evidence are that the compound is ACIDIC, burns yellow/orange (sodium indication), and forms a white precipitate with AgNO3"

Sodium nitrate is fairly soluble, so it will not form a precipitate, so did the solution form a precipitate with silver ions or not?

You don't have any acidic sodium salt (unless the sodium phosphate is NaH2PO4) that will form a precipitate with silver. The only acidic salt of sodium you have is the bisulfate.

B:
The basic (and not reducing) sodium salts you have are: phosphate, borate, citrate, oxalate, carbonate (not including acetate).
Test it with acid, if bubbles are formed, it is the carbonate.

We re-did the test today with the AgNO3 and we did observe a white precipitate for unknown A. We must've either misread, or goofed the test before, because it is a pretty clear white precipitate.

We will also re-test the pH tomorrow, there is probably a good chance it is basic, and seems logical that it would be, so we will re-test that tomorrow.

As far as B goes we will test it with an acid (again) tomorrow. Now that I think about it, when we tested it with H2SO4 it DID form bubbles eventually, but we were so fixated on the potential smell of vinegar we did not make a note of it...it did have an relatively strong odor to it, but may not have been vinegar. By bubbles forming, do you mean as in bubbles boiling at the top of the liquid, or more at the bottom. I remember observing bubbles forming at the bottom of the liquid..

Thanks for the help guys!!! I feel like we're really close to getting this nailed!

Mixell - 7-6-2012 at 15:34

Add the solid to H2SO4, it should fizz and bubble vigorously, you will not be able to miss it!
If the solution of A is neutral (low molar concentration), it can still be sodium acetate if there is an unmistakable smell of vinegar.

[Edited on 7-6-2012 by Mixell]

a.dunn.wa - 7-6-2012 at 16:02

Quote: Originally posted by Mixell

Add the solid to H2SO4, it should fizz and bubble vigorously, you will not be able to miss it!
If the solution of A is neutral (low molar concentration), it can still be sodium acetate if there is an unmistakable smell of vinegar.

[Edited on 7-6-2012 by Mixell]

Hmmm...when we added unknown B to H2SO4 it didn't seem to fizz and bubble vigorously, but we will try it again tomorrow! If it doesn't fizz and bubble vigorously, what other tests could we perform to figure out which one it is?

Mixell - 7-6-2012 at 18:09

If it did form a precipitate with AgNO3, then you can try to do some titrations (with appropriate indicators) do determinate what kind of sodium salt that is.
If the pH is neutral, it can be the sodium chloride/iodide/bromide, to determinate exactly which, you will need to oxidize he halides and check which is which (take a look at one of my previous replies to your thread).

a.dunn.wa - 9-6-2012 at 11:37

Well, we turned our lab in on Friday. We got 3 of the 4 right. Just in case any of you are interested these are the correct results:
Unknown A = sodium acetate
Unknown B = sodium nitrite
Unknown C = tin ii chloride
Unknown D = iron ii sulfate

We got A, C, and D completely right and we got 1/2 credit on B for getting the first half right (pretty sweet deal). We ended up getting a 95% and we really owe it to you guys for all your help/assistance.

Hexavalent - 9-6-2012 at 12:13

You're welcome, glad you got such a good grade! Congrats!