Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Problem of the Week Archive => Topic started by: Borek on July 30, 2012, 01:01:00 PM
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When a 1.22 L reaction vessel made of glass was filled with a dry, corrosive gas, its weight increased by
10.2 13.6 g (edit: data corrected) (temperature in the lab was 21 °C). After adding a single drop of water reactor was closed with a manometer. After several hours pressure inside the reactor increased by exactly 50% and the inside surface of glass became matted, with a slight yellow tint. Yellow tint disappeared after the reactor was opened and flushed with a concentrated solution of NaOH.
Name the gas.
Note: I am in a vacation mode, preparing questions in memory and drawing equations on the beach sand*. Chances that I am mistaken are even higher than in a typical POTW.
*An utter lie, no sand here, only rocks. It makes the preparations even more difficult.
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so far:
corrosive gas (A) + water = acid?? (yellow) + gas B
My initial thought is that the corrosive gas maybe a Nitrogen gas.
with the data available, and assuming that the initial pressure (P1) is caused by the gas A only, and the final pressure (P2) is caused by the gas B (product) only:
I have that: P2 = P1X3/2. Since Temp. an volume are constant=> applying the ideal gas law I got that:
Moles B/moles A = 3/2.
I didn't go further since this maybe completely wrong --feedback??? ???
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Assume glass is mainly SiO2.
Nitrogen is not corrosive, it is almost completely inert.
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Assume glass is mainly SiO2.
Nitrogen is not corrosive, it is almost completely inert.
sorry I meant a gas containing N element (an oxide like NX0Y)
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is it HF?
since it reacts with silica when dissolved in water to form fluorosilicic acid (colorless to light yellow)
SiO2(s) + 6 HF(aq) → H2SiF6(aq) + 2 H2O(l)
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Come on, you can do better than that. You have not even tried to check if your guess fits information given.
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Come on, you can do better than that. You have not even tried to check if your guess fits information given.
Negative. I actually tried and it didn't work :'(. But I was not sure if I did it right as well. Now I know I did it right..so I won't post again until I have something that fits 100% with the info given- particularly the pressure, I can't think on something that fits with that.
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Extended for another week.
You are right about HF reacting with SiO2. But HF is not the original gas.
What can you calculate using given information about weight?
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well - I can calculate the density of the gas
d=m/v= 10.2/1.22 = 8.36 g/L
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okay- I finally found a gas that fits with:
-pressure change
-color
but still it doesn't fit with the density (I am comparing at room temp. and 1 atm pressure, from online tables)
if I find a gas that fits with the density---I'll post everything at once...
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okay-I won't have time to continue with this problem for a few days, so I post what I tried if it helps to another person:
I tried with WF6, is a corrosive gas, density about 12 (that doesn't match) but it reacts with water to give:
WF6 + 3H20 = WO3 (yellow, dissolves in NaOH) + 6HF
Then HF produced reacts with SiO2:
4HF + SiO2 = SiF4 + 2H20
according to stoichiometry I got that, for each mol of WF6 that reacts, we get 3/2 moles SiF4 gas, that would give me the 50% increase in pressure.
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density about 12 (that doesn't match)
Think it over.
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density about 12 (that doesn't match)
Think it over.
well since I saw the problem I wondered about at which pressure we were working, but I was afraid to ask since that could be a hint ;D
I am thinking that if we are talking about a low pressure reaction vessel that can lower the gas density to 8.36 (not sure on this- I must say I don't remember much about under pressure vessels)
It's the only I can think of so far.
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You can assume pressure around 1 atm.
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You can assume pressure around 1 atm.
ok so molar mass gas = 201.79 g/mol
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You can assume pressure around 1 atm.
ok so molar mass gas = 201.79 g/mol
No. But I am not able to reproduce the correct answer either :'(, and as I calculated it in the Dalmatino bar using a napkin, I have no notes to consult.
Trick is, 10.2 g is not a weight of the added gas - it is the increase of the weight, so you should take buoyancy into account. As the volume is 1.22 L, and the air density is around 1.2 g/L, buoyancy is around 1.22*1.2=1.46 g - so the weight of the gas is around 10.2+1.4=11.6 g. That gives molar mass of around 230.
It should be 15.05-1.22*1.2=13.6 g, I am changing the wording of the original question. Sorry about that.
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You can assume pressure around 1 atm.
ok so molar mass gas = 201.79 g/mol
No. But I am not able to reproduce the correct answer either :'(, and as I calculated it in the Dalmatino bar using a napkin, I have no notes to consult.
Trick is, 10.2 g is not a weight of the added gas - it is the increase of the weight, so you should take buoyancy into account. As the volume is 1.22 L, and the air density is around 1.2 g/L, buoyancy is around 1.22*1.2=1.46 g - so the weight of the gas is around 10.2+1.4=11.6 g. That gives molar mass of around 230.
It should be 15.05-1.22*1.2=13.6 g, I am changing the wording of the original question. Sorry about that.
no worries :)