[Schrödinger]

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:


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.

[renge ishyo]

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.

[billnotgatez]

dropped it into a bowl of water

Was it truly water or did the water have something in it  - i.e. HCl

he bubbles caught fire and gleamed with the most beautiful crimson flame I have ever seen

The flame color is interesting.

[UG]

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  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?

[billnotgatez]

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.

[Schrödinger]

@ 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?

[zxt]

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.

[Borek]

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.

[billnotgatez]

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

[zxt]

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.

[zxt]

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.

[billnotgatez]

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

[zxt]

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  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.

[zxt]

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)

[zxt]

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.