Weird combustion

[Martoman]

A mixture of hexane and oxygen was placed in an evacuated vessel at 300°C. The pressure
inside the vessel was 80 kPa. After ignition by a spark, the mixture reacted completely to
form CO2, CO and water vapour. The contents of the container were returned to 300°C and
the pressure was found to be 130 kPa.
Which of the following equations best describes the reaction that took place?
A. C6H14(g) + 8O2(g) → 3CO(g) + 3CO2(g) + 7H2O(g).
B. C6H14(g) + 7O2(g) → 5CO(g) + CO2(g) + 7H2O(g).
C. 2C6H14(g) + 15O2(g) → 8CO(g) + 4CO2(g) + 14H2O(g).
D. 2C6H14(g) + 17O2(g) → 4CO(g) + 8CO2(g) + 14H2O(g).

For this one I did it visually so like the only thing that changed was products being formed which means an increase in volume must lead to the increase in pressure so what im looking for as n is proportional to V BUT it is also proportional to pressure.

So B? I don't have answers *sigh*

[ghadz7]

yeah I got B too

[Stroodle]

Yeah, I remember this question. I just figured the same (that n is proportional to P) which gives you B, and that was the correct answer, if I remember correctly.

[Martoman]

this invovled way too much thinking.

[ghadz7]

LOL! I added the coefficients on the right ( and coefficients on the left (13). Took me a three second

[Martoman]

Yeah. Getting to that point where you realise that.

[stonecold]

These are the workings.  I have no idea how this question works.

Q20 B According to pV = nRT, if the volume of the vessel is constant and the
temperature after reaction is allowed to return to the initial temperature, the
pressures before and after the reaction are proportional to the numbers of mole of
reactants and products respectively.
Since p(after reaction) / p(before reaction) = 130 / 80= 1.625

Hence n(products) / n(products) = 1.625.

Check each alternative
A. n(products) / n(products) = 13 / 9 = 1.444
B. n(products) / n(products) = 13 / 8 = 1.625
C. n(products) / n(products) = 26 / 17 = 1.529
D. n(products) / n(products) = 26 / 19 = 1.368

[stonecold]

how do you know the volume is constant?

[kyzoo]

Because its all done in a container of fixed volume

[stonecold]

okay, thanks kyzoo.  i just reduce everything to basic cross multiplication haha.

8 mol (initial) / 80 kPa = x mol (final) / 130 kPa

x = 13 mol

you can try it out for all of them, it only gives the right answer for B

[Martoman]

ROFL your simplifying to ratios is golden coldie.

[stonecold]

The day ratios don't work for me, I'm epically screwed.

Interestingly, the last two VCAA exams have had a whole question where all you had to do was set up a simple cross multiplication/ratios and solve for the unknown.  I love it!

Please have this this year...

And I now love AAS/UV-Vis calculations... all it is is ratios

[Martoman]

Then there will be the day where it gives you a non - linear one. Then the whole state is screwed.

[stonecold]

Yeah.  For absorbance you can have non-linear, but with stuff like taking a solution from one place and then diluting it, it is all x-multiplication

[Martoman]

You....forget.... this is the state of victoria.......