Heat of Combustion Question (1 Viewer)

mR sinister

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Wooh :) got 3.8g

overall, i think i aced core, maybe lost 2 marks in equilibrium question and lost 1 mark for nitrogen in bread
 

vpa2891

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Does anyone remember the calculation in NH3 question? I totally forget it :)
 

sam5

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The Bread Question?

It was 1.77%.

And yes...i did get that question right...unlike the bloody HoC lol.


btw, why was phenolphthalein a suitable indicator? Was it cos both 'vertical line parts' were at about the same pH values?
 

Gibbatron

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No it was because phenolpthalein has a significant colour change of about 8-10 pH, which was in the range of both titration equivilence points
 

kanux

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Do you remember the question ? :)
nope I don't sorry, well not the figures.

1. Writing the 2 equations to represent the two reactions

2. Find the excess moles of HCL

3. Calculate the percentage mass of NH3 in the bread

I think the bread was 2.8grams?

yer sorry, I don't remember the concentration or volume used D:
 

4theHSC

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No it was because phenolpthalein has a significant colour change of about 8-10 pH, which was in the range of both titration equivilence points
lol I'm picky about this one lol, its endpoint not equivilance, equivalence is when there is the same number of moles in both reactants, endpoint is when it changes colour
 

annabackwards

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No it was because phenolpthalein has a significant colour change of about 8-10 pH, which was in the range of both titration equivilence points
I wrote that too but i swear Acid 1's equivalence point was pretty much 7 T.T
 

gurmies

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I did the same thing
Me too =] Hi5 for smart pplz method xD

@ Anna, I noticed the same thing. Then I realised that it's the inflexion we're after, and not so much the point of equivalence.
 
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sam5

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lol I'm picky about this one lol, its endpoint not equivilance, equivalence is when there is the same number of moles in both reactants, endpoint is when it changes colour
i didnt use the terms 'equivalence point' or 'end point' (i know what they are, but i didnt use them, shit) but i did mention that phenolphthalein changes pink at about pH 10, and thats where both titrations were raising in pH and all hydrogen ions had been neutralised.

I hope thats enough (its only 1 mark).
 

Gibbatron

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lol I'm picky about this one lol, its endpoint not equivilance, equivalence is when there is the same number of moles in both reactants, endpoint is when it changes colour
I though endpoint was when the titration stopped and the colour change had already occured, not the point at which it occured. I'm known to be wrong so i'll take your word for it.
 

vpa2891

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nope I don't sorry, well not the figures.

1. Writing the 2 equations to represent the two reactions

2. Find the excess moles of HCL

3. Calculate the percentage mass of NH3 in the bread

I think the bread was 2.8grams?

yer sorry, I don't remember the concentration or volume used D:
I dont remember wat the question was...why everyone posting % mass of N instead of NH3
 

boxhunter91

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2,1,2,2,2? i think.
a) Write equations
b) ? Maybe i added an extra question
c) Moles
d) Excess Moles
e)% Mass
 

EvoRevolution

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Tell me if i did this wrong.

Heat = -0.21 x 4.18 x 10^3 x 65

= 57.057 KJ

I then divided this value by 2, because it said half the heat was lost to the environment.

Therefore, Heat = 28.5285

As HoC = Heat on moles

Therefore ; 1367 (given) = 28.5285 divided by m/(12.01 x 2 + 1.008 x 6 + 16)

Mass of Ethanol = 0.96 g (2.d.p.)
exactly what i got... but i rounded of to 1.00g
 

boxhunter91

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The question isnt really fair.
It said the molar heat of combustion is 1367kj/mol.
But in the question goes exactly half of its heat of combustion is lost.
So that means its mCAT is over two? doesnt it??
 

Aerath

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The question isnt really fair.
It said the molar heat of combustion is 1367kj/mol.
But in the question goes exactly half of its heat of combustion is lost.
So that means its mCAT is over two? doesnt it??
What do you mean?

If I did 1g to make 50J of energy, but 50% is lost to surroundings, that would mean that I would need 2g to make that 50J of energy.

By your logic, I'd only need 0.5g to make 50J, which is just wrong. :p
 

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