If we wish to find the molar mass of a pure solid organic compound, which measurement would be most useful and why?
A)melting point of the solid
B)melting point depression of the mixture of the solid with 1,4-dichlorobenzene
C)Heat of combustion
D)solubility in water
And what do you think?
I think B is most probably not the answer,we don't know whether they will react.
I think A may be the answer,so then we can compare it with the other compound series and try performing the reactions or identifying tests of that series(MAYBE)
No I think C is the answer.We can find the heat needed and the O2 moles ndeded,and we can measure the mass of the given sample.
And if we could collect all the CO2 emitted so that we can find the Carbon and Oxygen percentages and.....
I don't think we can answer this unless we know how accurate the molar mass determination must be made; i.e., 5-10% or much more accurate like <1% error. Here are my thoughts to go with yours.
I don't think A is a possibility and I'm confused with your first statement about A being compared to the "other" compound. There is nothing in the problem about "another" compound. If you take a good melting point the best you can do is look up that m.p. in a set of tables and HOPE you can find a compound that has that m.p. My experience is that you must look for a "range" in the m.p. and more than likely you will find SEVERAL compounds within that range. Of course that leads to several (even two is one too many) so you really are up the creek with A.
I think B is a possibility.
dT = Kb*molality. You will measure dT and you have Kb so you calculate m. Then m = mols/kg solvent. You know m and kg solvent so you calculate mols. Then mols = grams/molar mass and you calculate molar mass from that. These calculations give rough estimates on the molar mass but probably not better than 2% at the best and maybe 8-10% on the other end. This procedure gives a very good "starting point" for the molar mass. For example, the molar mass may be 50 and this procedure might give something like 54 or so.
You know d can't be right. Solubility just isn't definitive enough to isolate one organic compound from the thousands of choices we have.
That leaves C. That has possibilities,too, as with B but I don't agree with you about how C might help (but I agree with some of what you say).
CxHy + O2 ===> xCO2 + yH2O + heat
dHo rxn = (x*dHCO2 + y*dHH2O)-(dHCxYy)
So we can calculate dHf CxHy and what do we do with it. The only thing I see is to look in a set of tables and find the dHf of a matching compound to identify CxHy. After identifying the compound we can calculate the molar mass by adding the atomic masses. We hope there is only ONE possible compound we can pick from that table. Then if we could determine %C and %H we might determine the empirical formula, BUT (and this is a big BUT), that is another measurement (actually two more) and according the way I read the problem we are looking for the ONE measurement that makes the most sense. For another think, who says the compound contains just C and H. Perhaps N, S, Cl, others?
In the end I think we go with B or C. B gives us an answer that is in the ball park; C we know we can measure dH VERY close but then we must HOPE we can find that compound listed in a set of heat combustion tables and we find just one such compound.
Yes, I'm long winded.
There are no mention about how accurate this molar mass determination should be made and yes that's what I were thinking about A and now I understand its not the most useful data.
And I think from B and C,C is the most possible answer.
And thank you very much for taking your time to explain this..
To find the molar mass of a pure solid organic compound, the most useful measurement would be the heat of combustion, which is option C.
The molar mass of a compound can be determined using the concept of stoichiometry, which involves the balanced equation for the combustion of the compound. In the process of combustion, the compound reacts with oxygen to produce carbon dioxide and water. The heat released or absorbed during this reaction is known as the heat of combustion. This value can be measured experimentally using a bomb calorimeter.
By measuring the heat of combustion and knowing the products formed (carbon dioxide and water) and their enthalpies of formation, it is possible to calculate the molar mass of the compound. This can be done by applying the law of conservation of energy and using the equation q = mcΔT, where q is the heat released or absorbed, m is the mass, c is the specific heat capacity, and ΔT is the change in temperature.
The melting point of the solid (option A) is not directly related to the molar mass of the compound and cannot be used to determine it accurately. The melting point depression (option B) involves the formation of a mixture with 1,4-dichlorobenzene and is not a direct measure of the molar mass of the compound. Solubility in water (option D) also does not provide direct information about the molar mass.
Therefore, the best option for determining the molar mass of a pure solid organic compound would be the measurement of the heat of combustion (option C).