[curiouscat]

I got some really really tough ones, but it all depends on the area people like.

Tough ≠ Elegant 

What are gmol and gm? How did you get them? Shouldn't it be 222 + 660*n has units of kJ/mol?

gmol = mol

gm = grams

kJ/mol is the same as kJ/gmol

[Rutherford]

y=x(J/mol)*1000(g)/M(g/mol), this would be then the formula Q=ΔH*n, but Q is expressed in J only, not in kg  .

"Thus, 1 mol of CnH2n+2 will produce 882 + (n-1)*660 = 222 + 660*n kJ of heat..."

222 + 660*n has units of kJ/gmol

(14n+2) has units of gm/gmol

(222+660n)/(14n+2) has units of kJ/gm

(222+660n)/(14n+2)*1000 has units of kJ/kg

Then it's not the formula Q=ΔH·n. It is Q=ΔH/M. While I don't completely understand how is this formula derived, I believe that it is concluded from the definition of the specific heat of combustion, which I am not sure of how it sounds. Thank you for helping me.
For me, tough=elegant  .

[Big-Daddy]

PS. Of all the problems you posted this was a particularly elegant one I thought.

It's interesting that per mol heat does't tend to a an asymptotic constant value but per kg does. In hindsight it is obvious but I'd never analysed it like that.

Wonder if this can be generalized to every extensive quantity.

Yes it is a lovely problem, what confuses me is converting from kJg^-1 to kJkg^-1, because this seems unnecessary and I rarely see answers in MJ anyway! On the whole it is a sweet problem though.

I got some really really tough ones, but it all depends on the area people like. As I am preparing for competition I am doing all the areas (organic, inorganic, analytic, physical...). I asked some tough problems that are still unsolved after many months (I have only numerical answers on my papers).

What are gmol and gm? How did you get them? Shouldn't it be 222 + 660*n has units of kJ/mol?

Which competition? And if you have lots of hard questions I would love to try them. Any type (provided they are high school-level, as my knowledge of analytical at undergrad level is shaky), as many as you can - I love these types of problems and would really appreciate it. If any are unanswered I would do my best to help you with them as well.

[Rutherford]

Just go through my earlier posts, there are many problems. Unsolved, that I could find, are:
http://www.chemicalforums.com/index.php?topic=59059.0 which I don't think that can be solved.
http://www.chemicalforums.com/index.php?topic=55769.msg201714#msg201714 the first one.
http://www.chemicalforums.com/index.php?topic=65293.0 this one I solved actually.

[Big-Daddy]

Just go through my earlier posts, there are many problems. Unsolved, that I could find, are:
http://www.chemicalforums.com/index.php?topic=59059.0 which I don't think that can be solved.
http://www.chemicalforums.com/index.php?topic=55769.msg201714#msg201714 the first one.
http://www.chemicalforums.com/index.php?topic=65293.0 this one I solved actually.

The first seems impossible (perhaps you can work towards a ratio but not an exact mass), the second I don't understand the wording of and the third seems fairly standard.

Where do you get these challenging questions, though?

[Rutherford]

They are mostly from my country olympiad and these can't be found in English. I also posted few from other olympiads (IChO, Bellarus, Poland, USA...)