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### Topic: Find chemical formula from boiling point elevation?  (Read 24122 times)

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#### nikita

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##### Find chemical formula from boiling point elevation?
« on: February 07, 2009, 03:52:00 PM »
Ive been working on this problem for a few days and cannot come up with an answer.  I think it is just a mistake in setting up the problem, probably in terms of the algebra.

A metal, M, of atomic weight 96 reacts with fluorine to form a salt that can be represented as MFx . In order to determine x and therefore the formula of the salt, a boiling point elevation experiment is performed. A 9.18 g sample of the salt is dissolved in 100.0 g of water and the boiling point of the solution is found to be 374.38 K.  Find the formula of the salt. Assume complete dissociation of the salt in solution.

here is what i did:
:delta: Tb = Tsoln - Tsolvent
K to C 101.38 = 374.38 - 273.00
:delta: Tb= 1.38 = 101.38-100.00

now, i have googled the question (of course after trying it myself and have found 2 answers calling the k constant of water 1.86.  I thought the k constant was .512 for boiling water.  please correct me if i am wrong.
:delta: Tb = kb * m
1.38 = .512m
1.38 = .512 [(9.18/M)/.100]  (.100 being the kg of water)

the mistake may be here?:
1.38 = 4.70/.100M (where M is the molar mass of the compound)
.138M = 4.70
M = 34.06

clearly the molar mass is not 34.06 because it cant even account for the metal M of 96g.  However, if i do the problem with 1.86 as constant k (even though im sure its the constant for freezing point) and round  :delta: Tb to 1.00 i come up with 170.75 as the molar mass of the compound.  but if i subtract 96 and divide by 19 for fluorine, i come up with MF4 but with 3.93 left over.  im fairly confused.  any pointers or clues as to where i am going wrong?

#### Borek

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##### Re: Find chemical formula from boiling point elevation?
« Reply #1 on: February 07, 2009, 06:34:44 PM »
Looks to me like you are omitting fact that salt is dissociated.

273.00 or 273.15?

0.512 it is.
« Last Edit: February 07, 2009, 06:49:36 PM by Borek »
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#### nikita

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##### Re: Find chemical formula from boiling point elevation?
« Reply #2 on: February 07, 2009, 08:19:19 PM »
ah, my math was ok, my reading of the question was not....(thanks)

yes, 273.15.  duh.

so.....instead of what i did before, how about i just find the molality right away and go from there, also not being as general with the molar mass of the sample?

1.23/.512 = 2.36
2.36/.1 = .236m

then setting the equations equal

.236/(x+1) = 9.18/(96+19x)     (x+1 for the dissociation....i know M is one, x for the F and molar mass of 96 +19x)
22.7 + 4.48x = 9.18x + 9.18
13.52 = 4.70x
x = 2.89 or 3
so it would be MF3?

#### Borek

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##### Re: Find chemical formula from boiling point elevation?
« Reply #3 on: February 08, 2009, 04:07:22 AM »
That's what I got too, no idea whether it is OK
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#### nikita

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##### Re: Find chemical formula from boiling point elevation?
« Reply #4 on: February 08, 2009, 02:59:37 PM »
it was correct, thank you for the help, Borek.