[ian123]

Is C3H7O2 an organic molecule? I think it's an anion and not a molecule. If it is an organic molecule please help me with the structure. My exam paper states that it is an organic molecule.

A solution of 1.781g of an organic molecule C3H7O2 in 100g of benzene freezes at a temperature 0.606K below the melting point of pure benzene. The cryoscopic constant for benzene is 5.11 K molal-1. Calculate the molecular weight of the molecule in benzene and comment on your result. State any assumptions that you have made and suggest a structure for the dissolved molecules.

Having done the calculation, here is my conclusion:
C3H7O2 has a molecular weight of 75g/mol
Molecular weight of C3H7O2 is 150.18g/mol
This answer gives double the molar mass we expected i.e two molecules form a single particle in solution in benzene. But what about the structure? Can you think of a possible structure?

[Dan]

Since no charge is given in the formula, we should assume it has no charge. As it stands, the neutral formula would have to be a radical (since the degree of unsaturation is 0.5).

This seems like an odd exam question, context would be useful. Maybe it's an empirical formula. Please post the entire question as it appears on the paper.
[Note: Original post modified to include this information after this message was posted]

There are several isomeric possibilities for C₃H₇O₂ - context needed (see above).

[ian123]

Is C3H7O2 an organic molecule? I think it's an anion and not a molecule. If it is an organic molecule please help me with the structure. My exam paper states that it is an organic molecule.

A solution of 1.781g of an organic molecule C3H7O2 in 100g of benzene freezes at a temperature 0.606K below the melting point of pure benzene. The cryoscopic constant for benzene is 5.11 K molal-1. Calculate the molecular weight of the molecule in benzene and comment on your result. State any assumptions that you have made and suggest a structure for the dissolved molecules.

Having done the calculation, here is my conclusion:
C3H7O2 has a molecular weight of 75g/mol
Molecular weight of C3H7O2 is 150.18g/mol
This answer gives double the molar mass we expected i.e two molecules form a single particle in solution in benzene. But what about the structure? Can you think of a possible structure?

This is the question and the answer I just do not know the structure of this molecule.

[DrCMS]

As Dan has said are you sure the question is not:

"A solution of 1.781g of an organic molecule with the empirical formula of C3H7O2 in 100g of benzene freezes at a temperature 0.606K below the melting point of pure benzene. The cryoscopic constant for benzene is 5.11 K molal-1. Calculate the molecular weight of the molecule in benzene and comment on your result. State any assumptions that you have made and suggest a structure for the dissolved molecules."

[ian123]

Yes, I am sure about what I wrote.

[ian123]

As Dan has said are you sure the question is not:

"A solution of 1.781g of an organic molecule with the empirical formula of C3H7O2 in 100g of benzene freezes at a temperature 0.606K below the melting point of pure benzene. The cryoscopic constant for benzene is 5.11 K molal-1. Calculate the molecular weight of the molecule in benzene and comment on your result. State any assumptions that you have made and suggest a structure for the dissolved molecules."

Yes, I'm sure about what I wrote.

[Babcock_Hall]

My hunch is that there is a typo in the problem; therefore, I suggest we focus on different information.  Can you think of a chemical explanation for the apparent doubling of molecular weight?

[Arkcon]

Everyone, including the O.P., should re-read this problem.  Yes, a molecule made of carbon is almost always organic.  But this problem uses coligative properties, depression of freezing point, to determine molecular weight.

Now, ian123:, its interesting that you wonder if its an anion, that is a pretty smart catch of yours.  If sucrose is dissolved in water, that's also an organic, but not an anion.  Acetic acid in water is an anion, and its freezing point depression becomes a more complex problem.  But an organic compound, dissolved in benzene, is not going to ionize, unless we're talking about a crown ether making purple benzene, and we're not.

So, start finding the formulas you need for freezing point depression, and lets see what molecular weight you can determine based on the lowering of freezing point.

[ian123]

I've already stated my answer in the forum. It says that make an assumption so I'm guessing that I have to get ignore the negative charge present. that's what I think, of course.

[Borek]

So, start finding the formulas you need for freezing point depression, and lets see what molecular weight you can determine based on the lowering of freezing point.

That was already done, and it produces almost exactly twice the formula mass.

I am baffled, something seems to be wrong.

[Borek]

I've already stated my answer in the forum. It says that make an assumption so I'm guessing that I have to get ignore the negative charge present. that's what I think, of course.

Anion in non polar solvent?