Lab setup about gas solubility

[bidiboom]

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Hi people,

Actually this is a question I am interested in conceptually. The point is not only the answer, but to understand the approach:

There is a diagram of a laboratory setup, a classical one to prepare and collect a gas heavier than air: there is a flask with a thistle tube over and some solution inside, another tube getting out of the flask and getting in a capped bottle beside the flask. The question is:

The diagram represents a setup that may be used to prepare and collect
A) NH3
B) NO
C) H2
D) SO3
E) CO2

The answer is CO2. But I am confused, since both NO, SO3 and CO2 are all heavier than air, and whats more CO2 is moderately soluble.

1) Why CO2, among all?

2) How can a gas which can dissolve in the solution in the flask can be collected in the bottle? This wont be an accurate collection, am I wrong?

3) Also if a gas is heavier than air, then how can it float up high in the flask and get in the tube thats used to collect the gas in the bottle? Doesnt a heavier-than-air gas have an inclination to stay right over the solution in the flask? In this setup the total gas produced in the flask cant be collected in the bottle, to me. Am I wrong?

This time its a pretty long and tiresome question.. sorry, but the question marks in my mind are those above.. if you can find time and will, I will be happy with whatever you can say.. I dont want you to teach me a whole concept, please dont misunderstand me.. I want to see your perspective.

Thank you..

edit: By the way, to my knowledge,
Air is 29gr/mol.
NO, 30gr/mol.
SO3, 80gr/mol (its soluble as well and creates H2SO4, so its ok not to pick this one)
CO2, 44gr/mol.

 

[cheesier]

The solution in the flask is the reacting mixture producing the gas, not water, so solubility doesn't really matter. The way it works is, gas is produced in the solution and it diffuses out of the flask, through the tube, and displaces the air in the adjacent flask because it is heavier.

As for how it reaches the other tube if it is heavier than air, it is diffusing down its concentration gradient to the lowest point.

 

[bidiboom]

The solution in the flask is the reacting mixture producing the gas, not water, so solubility doesn't really matter.

If I could be able to rule out this point, it wouldnt matter. But some questions are demanding to take into consideration solubility point as well.. say for instance, there are such questions that eliminate NH3 just because its soluble in water, or lets say, polar solvents.

 

[Pisiform]

If I could be able to rule out this point, it wouldnt matter. But some questions are demanding to take into consideration solubility point as well.. say for instance, there are such questions that eliminate NH3 just because its soluble in water, or lets say, polar solvents.

You would only take solubility into account if you are collecting gas over water. In that case, gas you are collecting should not be soluble in water. As the gas collects it will displace water and you can measure the volume and from you can use PV=nRT to find out number of moles.

But if you are doing downward delivery, then you don't have to worry about their solubility in water because its not there.For toxic gases like SO2 etc, you shouold use fume cupboard

 

[bidiboom]

You would only take solubility into account if you are collecting gas over water. In that case, gas you are collecting should not be soluble in water. As the gas collects it will displace water and you can measure the volume and from you can use PV=nRT to find out number of moles.

But if you are doing downward delivery, then you don't have to worry about their solubility in water because its not there.For toxic gases like SO2 etc, you shouold use fume cupboard

Good tip, thank you Pisiform.. as for density, do you have anything to say? Regarding the options, there are more than one gas which is denser than air:
Air is 29gr/mol.
NO, 30gr/mol.
SO3, 80gr/mol
CO2, 44gr/mol

Why shall I choose CO2, and not NO or SO3?

 

[cheesier]

Not sure if this is the best answer, but CO2 is prepared via combustion reactions at low temperatures. Assuming that the NO and SO3 are made in the reaction mixture in the flask, it would have to be extremely hot (wikipedia says 500 C +) so downward delivery wouldn't be feasible.

 

[Pisiform]

Good tip, thank you Pisiform.. as for density, do you have anything to say? Regarding the options, there are more than one gas which is denser than air:
Air is 29gr/mol.
NO, 30gr/mol.
SO3, 80gr/mol
CO2, 44gr/mol

Why shall I choose CO2, and not NO or SO3?

Once again, in downward displacement solubility won't matter so all the gases that are heavier than air NO, SO3, CO2 can be collected by this method. I don't know how CO2 has to be special ...