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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Schrödinger on August 16, 2009, 04:02:23 PM

Title: IChO preparatory problem 2008....Superb question!
Post by: Schrödinger on August 16, 2009, 04:02:23 PM
Ok, this question is an IChO preparatory problem, 2008, a very nice one indeed.

This is only an extract from the original question.
Please try to read the entire question, it is a beautiful one.

This is the part i have doubts in. Here goes:

...’It is far better to keep the contents in mineral oil,’ Holmes explained and showed me a bottle. ’This will keep it safe from air but also makes it more flammable.’
The yellowish liquid in the bottle covered a few thumb-sized pellets.
’Is this a dangerous poison?’ I asked.
’Not at all, Watson. Have you ever seen a poison in so big a pellet? It would hardly be healthy to swallow, but that is not the point. Now look at this.’
He took out a pellet, dried it with great care, and dropped it into a bowl of water. Instead of slowly dissolving or sinking, the pellet began a strange dance on the surface of the water, hissed ominously, gave out bubbles and some malodorous product. The acrid fumes took me by the throat and set me coughing.
’Holmes, this will kill us both,’ I screamed.
’You should have seen the reaction with hydrochloric acid. Anyway, I told you it is not particularly poisonous,’ said Holmes coughing. With dramatic suddenness he struck a match, and as he held the match nearer, the bubbles caught fire and gleamed with the most beautiful crimson flame I have ever seen.
’Magnificent, is it not? One ounce of this substance when reacting with water or hydrochloric acid gives more than three cubic feet of gas. To be precise, 3.068 cubic feet at 80.0 degrees and atmospheric pressure.’
’You measured this?’ I cried.
’Of course I measured it,’ said Holmes with an impatient gesture. He took a small bottle labelled phenolphthalein and put a few drops of its contents into the bowl of water, which turned pink immediately, its colour resembling the gleam of the flames.
’Is this why this substance is so precious?’



I tried to solve the question and from this part of the question i have posted, i have come to the conclusion that the pellets are LiH.

First, i tried to do the calculation bit, and then i tried to substitute certain compounds which i thought would fit the bill. This, i guessed using some 'chemical intuition'.

And  LiH  perfectly agrees with the values...yippee!!! :)

The eqn that i obtained is thus:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.forkosh.dreamhost.com%2Fmimetex.cgi%3F%7B+%5Cmathrm%7BLiH%7D+%2B+%5Cmathrm%7BH_2+O%7D+-%26gt%3B+%5Cmathrm%7BLiOH%7D+%2B+%5Cmathrm%7BH_2%7D+%7D&hash=f501e9fe2c4de11ce827ea4b9c443f8b)

My doubts arouse from this point onwards:
What is that malodorous product given in the question?
- It's certainly not Hydrogen gas , as it is odourless. I don't know about LiOH...

What are those acrid fumes?
-Again, it can't be hydrogen...

 and What about the bubbles?What are they, and why do they burn with a crimson flame? Does it have anything to do with the flame test for Li???

Please help me out!!


Note : Temperature in Fahrenheit.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: renge ishyo on August 16, 2009, 11:40:16 PM
Hiya Shrodie,

I don't know what solid lithium hydroxide smells like if you inhale it, but one look at the materials safety data sheet for it reveals that inhaling it can indeed cause someone to choke or cough as the story describes. It can also cause death if too much is inhaled as well. Those things are consistent with the story at least. But it doesn't vaporize easily and it is not combustible so it would be the malodorous product as opposed to the bubbles.

My guess is that the bubbles coming out of solution would likely be the hydrogen gas as it would be released out of solution during the course of the reaction. Hydrogen gas is very flammable and would react with the oxygen in the air as it emerges from the beaker if a match was lit to start the reaction.

Seems plausible, but parts of the story are unclear so I am not sure either.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 16, 2009, 11:52:40 PM
Quote
dropped it into a bowl of water
Was it truly water or did the water have something in it  - i.e. HCl
Quote
he bubbles caught fire and gleamed with the most beautiful crimson flame I have ever seen
The flame color is interesting.

Title: Re: IChO preparatory problem 2008....Superb question!
Post by: UG on August 17, 2009, 01:05:15 AM
I tried to solve the question and from this part of the question i have posted, i have come to the conclusion that the pellets are LiH.
Certainly is an interesting question, I Googled it and this bit springs to mind:

..so I re-dissolved it in water and added some hydrofluoric acid until the colour of phenolphthalein was gone. I boiled away the water again, and drying the white residue was not a problem this time. Its mass was precisely three and one eighth ounces. Three and one eighth. Do you see, Watson?’

You would expect the mass of LiF to be 3.25 times greater than that of lithium hydride if the molar mass of LiH was 8 grams mol-1.

LiH + HF  :rarrow: LiF + H2

But three and one eighth ounces is only 3.125 times greater. Therefore, the conclusion is that Li in LiH exists as 6Li instead of 7Li (fluorine-19 is the only stable isotope), so H should actually be Deuterium. And when they asked: What could it possibly have been intended for?
My guess: nuclear weapons?
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 17, 2009, 01:53:16 AM
How easily I am tricked - I thought it was calcium carbide or lithium carbide.
I guess that is what I get for not reading the problem properly.

Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Schrödinger on August 17, 2009, 02:08:41 PM
@ UG : wow!!! Never thought of isotopes.
I mean i'm like : isotopes? Whhhaaaaaatt!!!!!!!!! How come it never occured to me!

Anyway, can someone tell me what the flame colour is indicative of? How come Lithium's flame test is answered? Or is it?
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 21, 2009, 07:59:52 AM
Here I calculated. P=1.013*105Pa, V=0.087m3, T=335.93K, R=8.314J/(K*mol), n=(p*V)/(R*T), so n=3.156mol, right? (1 ounce= 20.35g, 1 feet=0.305m)

And let's assume that one mole compound creates one mole hydrogen( I think the gas is just H2), and do the math:20.35/6.31=M/2, I get M=6.45g/mol, but it isn't 6LiH or 7LiH. Something wrong with my calculation? If any, please point out and I do think the color of the flame indicates the existence of Lithium because Lithium's flame test color is crimson.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Borek on August 21, 2009, 09:19:57 AM
3.156 doesn't look good to me.

Check out the temperature units.

Also, no idea what ounce should be used, but neither I know is close to 20 g.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 21, 2009, 09:42:13 AM
1 ounce Avoirdupois  = 28.35 grams

I have not done the math but I thought
UG
was suggesting

6Li2H

I am still puzzled by the flame color too
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 21, 2009, 09:50:41 AM
Oh yes, thank you Borek! I think the ounce here refers to "International avoirdupois ounce" which equals to 28.34952g and the temperature should be 299.75K, then the M recalculated is almost 8 which indicates it's LiH.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 21, 2009, 09:54:16 AM
1 ounce Avoirdupois  = 28.35 grams

I have not done the math but I thought
UG
was suggesting

6Li2H

I am still puzzled by the flame color too

Yes, it may be 7Li1H or 6Li2H. One thing is sure that the color of flame due to element Li.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 21, 2009, 10:14:43 AM
Since this is a story maybe they got the color wrong.

Or

There is some sort of mantle effect with the hydrogen flame

Or

Synergistic effect when burning hydrogen and lithium


Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 21, 2009, 10:27:15 AM
I tried to solve the question and from this part of the question i have posted, i have come to the conclusion that the pellets are LiH.
Certainly is an interesting question, I Googled it and this bit springs to mind:

..so I re-dissolved it in water and added some hydrofluoric acid until the colour of phenolphthalein was gone. I boiled away the water again, and drying the white residue was not a problem this time. Its mass was precisely three and one eighth ounces. Three and one eighth. Do you see, Watson?’

You would expect the mass of LiF to be 3.25 times greater than that of lithium hydride if the molar mass of LiH was 8 grams mol-1.

LiH + HF  :rarrow: LiF + H2

But three and one eighth ounces is only 3.125 times greater. Therefore, the conclusion is that Li in LiH exists as 6Li instead of 7Li (fluorine-19 is the only stable isotope), so H should actually be Deuterium. And when they asked: What could it possibly have been intended for?
My guess: nuclear weapons?



Actually I think the whole course is this:

First LiD+D2O :rarrow:LiOD+D2, then add HF
LiOD+HF :rarrow:LiF+HDO

and the mass of LiF is exactly 88.59g.

H2O and D2O are both right. So is HDO. Any way H2O is most common.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 21, 2009, 11:04:20 AM
Quote
and some malodorous product. The acrid fumes took me by the throat and set me coughing.

Here the fumes is NH3

6Li(s) + N2(g) :rarrow:2Li3N(s) this reaction happens under room temperature.

then
 
Li3N (s) + 3 H2O (l) → 3 LiOH (aq) + NH3 (g)
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 21, 2009, 11:11:21 AM
Quote
and why do they burn with a crimson flame

Here the mechanism I think. When a bubble breaks, it splashes many tiny drops of LiOH solution and when these drops touch fire, they show crimson color due to Li inside.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Schrödinger on August 21, 2009, 02:36:44 PM
@zxt : You said the fumes were NH3, which is formed from Li. But where did you get that solid Li as such? Isn't Li present as LiH and doesn't it become LiOH? So, how were you able to use Li metal?
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 21, 2009, 09:22:04 PM
Ennn…… LiH decomposes at 850 deg centigrade, and the burning temperature of hydrogen can reach that value, so maybe the source of solid Li comes from that. I have a gut feeling that the gas is ammonia.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 22, 2009, 03:24:08 AM
If NH3 were produced would it  mess up the gas volume measurement?
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 22, 2009, 04:03:58 AM
Are we in agreement that we get about 3.53 mol of ideal gas?
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: UG on August 22, 2009, 04:14:04 AM
I'm in  ;D
3.531 moles
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Borek on August 22, 2009, 04:17:30 AM
Here you go...
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 22, 2009, 04:28:05 AM
If NH3 were produced would it  mess up the gas volume measurement?

Yes, the gas is not any pure but it doesn't matter to calculate the molar mass of LiD, because the molar ratio between LiH and H2 or that between Li3N and NH3 are both 1:1, so we can suppose that the gas is just H2(even we don't know what the gas is, just knowing the molar amount of the gas, we can also identify the molar mass of the compound), meanwhile ammonia is very soluble in water, the gas measured in the story may also just be H2.

As for compound gas of n integrants, it obeys rule: PV=nRT, here P=p1+p2+……Pn, n=n1+n2+……nn. So if ammonia mixed in the gas, it doesn't affect the whole P and n, the result doesn't change.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Sam (NG) on August 22, 2009, 04:39:34 AM
I think that UG might be right with Lithium-6-Deuteride, especially when you consider the last line from the question is this:

’I do not wish to make a mystery,’ said he, laughing. ’The matter is elementary; simplicity itself. You remember our little adventure with Professor Urey?’

If you look up Harold C. Urey, you find out that he was involved in all sorts of nuclear naughtiness.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Borek on August 22, 2009, 04:47:40 AM
If you have ever dissolved NaOH (strong base) - there are acrid fumes. My take is that somehow solution gets sprayed (atomized) and this spray - as it is caustic - is acrid. I suppose that's what they refer to.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 22, 2009, 04:55:35 AM
Thank you Sam

I thought i had read all the Doyle stories and could not remember  Professor Urey

By the way, I went to the internet and got the whole problem which helped me understand some of the posts
I assume the original post was truncated for space or copy right issues.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Sam (NG) on August 22, 2009, 05:06:00 AM
Thank you Sam

I thought i had read all the Doyle stories and could not remember  Professor Urey

By the way, I went to the internet and got the whole problem which helped me understand some of the posts
I assume the original post was truncated for space or copy right issues.


I only found the end bit by seeing the original problem on google too.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: DrCMS on August 22, 2009, 05:23:05 AM
Why do you think it is LiH rather than just Li metal?
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 22, 2009, 05:33:12 AM
DrCMS

When you look at the whole problem some things become self evident

Problem 1
During a new construction at 221 Baker Street, an amazing discovery was made. A small cabinet was found containing previously unknown documents. They revealed that Dr. Watson kept notes about his adventures with Mr. Sherlock Holmes into the 1950s. An interesting, but incomplete story read as follows:

....and was able to spring into a cab and drive to Baker Street, half afraid that I might be too late to assist at the dénouement of the little mystery. I found Sherlock Holmes alone, however, half asleep, with his long, thin form curled up in the recesses of his arm-chair. A formidable array of bottles and test-tubes, with the pungent smell of hydrochloric acid, told me that he had spent his day in the chemical work which was so dear to him. It was obvious to me that my companion had already examined the carefully closed metal box we had found in a recess behind a sliding panel just above the right bell-pull in poor Browning's sitting-room.
'No doubt the murderers were after this box,' said he. 'They searched the house from cellar to garret. I would have been at a loss myself if I had not met Irene Adler some time ago...'
My attention was captured by the open box on the mantelpiece. It was empty.
'It is far better to keep the contents of the box in mineral oil,' Holmes explained and showed me a bottle. 'This will keep it safe from air but also makes it more flammable.'
The yellowish liquid in the bottle covered a few thumb-sized pellets.
'Is this a dangerous poison?' I asked.
'Not at all, Watson. Have you ever seen a poison in so big a pellet? It would hardly be healthy to swallow, but that is not the point. Now look at this.'
He took out a pellet, dried it with great care, and dropped it into a bowl of water. Instead of slowly dissolving or sinking, the pellet began a strange dance on the surface of the water, hissed ominously, gave out bubbles and some malodorous product. The acrid fumes took me by the throat and set me coughing.
'Holmes, this will kill us both,' I screamed.
'You should have seen the reaction with hydrochloric acid. Anyway, I told you it is not particularly poisonous,' said Holmes coughing. With dramatic suddenness he struck a match, and as he held the match nearer, the bubbles caught fire and gleamed with the most beautiful crimson flame I have ever seen.
'Magnificent, is it not? One ounce of this substance when reacting with water or hydrochloric acid gives more than three cubic feet of gas. To be precise, 3.068 cubic feet at 80.0 degrees and atmospheric pressure.'
'You measured this?' I cried.
'Of course I measured it,' said Holmes with an impatient gesture. He took a small bottle labelled phenolphthalein and put a few drops of its contents into the bowl of water, which turned pink immediately, its colour resembling the gleam of the flames.
'Is this why this substance is so precious?'
'Not really,' murmured Holmes. 'The Powers of Evil created these pellets, Watson, or I am very much mistaken. The murder of Browning was nothing but a trifle in this case.'
Half crazy with fear, I looked at the marble-like pellets in the bottle.
'I do not really understand, Holmes.'
'I made accurate measurements. I dissolved exactly one ounce of this substance in water, then boiled away the water. The remaining white solid I could not dry completely, so I re-dissolved it in water and added some hydrofluoric acid until the colour of phenolphthalein was gone. I boiled away the water again, and drying the white residue was not a problem this time. Its mass was precisely three and one eighth ounces. Three and one eighth. Do you see, Watson?'
'I am still in the dark,' I answered with some embarrassment.
'I do not wish to make a mystery,' said he, laughing. 'The matter is elementary; simplicity itself. You remember our little adventure with Professor Urey?'

The notes end here. Sherlock Holmes uses imperial units of measurement: 1 foot equals 30.48 cm, 1 ounce is 28.350 g, the atmospheric pressure has been constant (101325 Pa) over the last few centuries. The temperature is measured in degrees Fahrenheit (°F): 0 °C equals 32 °F, whereas 100 °C is 212 °F.

Help Watson figure out what was in the box. What could it possibly have been intended for?

Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 22, 2009, 05:44:30 AM
If you have ever dissolved NaOH (strong base) - there are acrid fumes. My take is that somehow solution gets sprayed (atomized) and this spray - as it is caustic - is acrid. I suppose that's what they refer to.

But it cannot be malodorous.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Sam (NG) on August 22, 2009, 05:55:40 AM
If you have ever dissolved NaOH (strong base) - there are acrid fumes. My take is that somehow solution gets sprayed (atomized) and this spray - as it is caustic - is acrid. I suppose that's what they refer to.

But it cannot be malodorous.

Nor can you make lithium nitride at room temperature as far as I am aware.  The way they used to synthesise it here was by dissolving the lithium in liquid sodium and heating to 450°C in a nitrogen atmosphere.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 22, 2009, 06:08:13 AM
I'm really confused now. What's this malodorous, acrid fumes on earth? I don't think the story made any mistake and this question remains the only one to me now.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 22, 2009, 07:18:08 AM
Dictionary dot com had several enteries

malodorous 
mal·o·dor·ous 
–adjective having an unpleasant or offensive odor; smelling bad: a malodorous swamp. 

Origin:
1840–50; mal- + odorous

Related forms:

mal·o·dor·ous·ly, adverb
mal·o·dor·ous·ness, noun
 
mal·o·dor·ous  (mal-o'd?r-?s)   
adj.  Having a bad odor; foul.
mal·o'dor·ous·ly adv., mal·o'dor·ous·ness n.

malodorous 

1850, from mal- "bad" + odorous.
 
 
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: Schrödinger on August 22, 2009, 10:43:58 AM
Guys, about the malodorous product, please refer to the first reply to the topic...Aren't we convinced with the malodorous product being LiOH?
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: zxt on August 22, 2009, 11:45:40 AM
Guys, about the malodorous product, please refer to the first reply to the topic...Aren't we convinced with the malodorous product being LiOH?


O.K. I agree.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: DrCMS on August 22, 2009, 01:35:07 PM
LiOH is not foul smelling but is not fun to breath in; it catches the back of your throat is a very distinctive way.
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 22, 2009, 04:54:49 PM
This is what WIKI has for flame test colors

Symbol Name Color
As Arsenic Blue
B Boron Bright green
Ba Barium Pale/Apple green
Ca Calcium Brick red
Cs Caesium Blue-Violet
Cu(I) Copper(I) Blue
Cu(II) Copper(II) (non-halide) Green
Cu(II) Copper(II) (halide) Blue-green
Fe Iron Gold
In Indium Blue
K Potassium Lilac
Li Lithium Red
Mn(II) Manganese(II) Yellowish green
Mo Molybdenum Yellowish green
Na Sodium Intense yellow
P Phosphorus Pale bluish green
Pb Lead Blue
Rb Rubidium Red-violet
Sb Antimony Pale green
Se Selenium Azure blue
Sr Strontium Crimson
Te Tellurium Pale green
Tl Thallium Pure green
Zn Zinc Bluish green
Title: Re: IChO preparatory problem 2008....Superb question!
Post by: billnotgatez on August 22, 2009, 05:00:19 PM
I guess Li Lithium Red could be considered crimson.

Quote
gleamed with the most beautiful crimson flame