Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Problem of the Week Archive => Topic started by: Borek on April 29, 2013, 05:20:52 PM
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1.000 g of a roasted salt decomposes producing three oxides - 0.878 g of a solid one, 0.0354 g of a liquid one and 0.0866 g of a gaseous one (at STP).
Determine the salt identity.
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I got Pb(OH)2·PbCO3 or (PbOH)2CO3, not quite sure :-\.
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(PbOH)2CO3,
Surely this can't exist ... That would mean the (PbOH)+ ion exists which I doubt.
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Okay, then maybe the first one.
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Borek when you say oxides do you just mean the salt contains oxygen in some form, or specifically the O2- ion? (Or can you not tell us?)
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Borek when you say oxides do you just mean the salt contains oxygen in some form, or specifically the O2- ion? (Or can you not tell us?)
No idea what you are asking about. Have you seen gaseous oxide containing O2-?
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Borek when you say oxides do you just mean the salt contains oxygen in some form, or specifically the O2- ion? (Or can you not tell us?)
No idea what you are asking about. Have you seen gaseous oxide containing O2-?
No, but then Raderford is suggesting a carbonate, which I would not normally class as an oxide. So by oxide you mean any salt containing O somewhere?
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No, but then Raderford is suggesting a carbonate, which I would not normally class as an oxide. So by oxide you mean any salt containing O somewhere?
Have you read the question?
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No, but then Raderford is suggesting a carbonate, which I would not normally class as an oxide. So by oxide you mean any salt containing O somewhere?
Have you read the question?
:-[ That was stupid of me.
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TlHCO3
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I didn't get the exact amounts you have posted, but very close.
2TlHCO3 :rarrow: Tl2CO3 + CO2 + H2O
Tl
Tl2CO3 = .883g
CO2 = .083g
H2O = .034g
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Tl2CO3 is not an oxide.
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Pb2(OH)2CO3
Pb2(OH)2CO3 :rarrow: 2PbO + CO2 + H2O
numbers are spot on this time.
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Raderford was first:
I got Pb(OH)2·PbCO3 or (PbOH)2CO3, not quite sure :-\.
No matter how you write it, this is the intended answer.
And I should slap myself again, as I have not checked details. Problem was used in a chemistry competition, and after solving and checking that the numbers are OK I assumed the compound is just a basic lead carbonate. Turns out it is not - so called white lead has a slightly different formula (2PbCO3·Pb(OH)2). It doesn't mean Pb(OH)2·PbCO3 doesn't exist, as there are several similar compounds, but I prefer problems with the obviously existing real solutions.
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Would anyone mind providing a link or a keyword for google where I can read how a problem like this is approached/solved?
Thank you
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It requires a lot of practice and trial and error. I usually encounter problems when solving tasks like this one.
Here you had a liquid oxide. The most famous liquid oxide is water, so you assume that it is water, you calculate its quantity and compare, e.g. with the gaseous oxide. If they got formed in 1:1 ratio, then the gaseous oxide has a molar mass of 44g/mol which corresponds to few gasses, but a good guess would be carbon dioxide (because when acidic or basic carbonate salts decompose, they can form solid oxide, water and carbon dioxide). You do the same for the solid oxide. Assume 1:1 ratio, and you will get a very big molar mass that doesn't correspond to any simple oxide I can think of. Then try the 2:1 ratio, and you will end with a molar mass of 223g/mol. Now you assume that there is one oxygen atom in the oxide, so the other element that composites the oxide has a molar mass of 207g/mol (Pb). Now you need to pack these oxides into one compound. As you see, many assumptions have to be made.