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Molar heat of combustion vs. Molecular weight. (1 Viewer)

How did you plot your graph ?

  • Plotted values as crosses and used line of best fit.

    Votes: 46 48.9%
  • Plotted values as crosses and used curve of best fit.

    Votes: 7 7.4%
  • Plotted values as crosses and did not use a line or curve of best fit.

    Votes: 4 4.3%
  • Plotted values as dots and used line of best fit.

    Votes: 25 26.6%
  • Plotted values as dots and used curve of best fit.

    Votes: 4 4.3%
  • Plotted values as dots and did not use a line or curve of best fit.

    Votes: 8 8.5%

  • Total voters
    94

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I plotted my molecular weight across the horizontal axis in 20 unit increments.
I plotted my molar eight of combustion across the vertical axis in 200 kJ/mol increments.
Unfortunately my line of best fit didn't even go through any of my plots (crosses)

My estimated heat of combustion for 1-propanol was 1700 kJ/mol ... I think that's way off ...
My reason why other than heat loss why the heat of combustion was less than the theoretical value was 'incomplete combustion may have occurred as a result of lower oxygen levels in the laboratory'.
 

Shrikar

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Was joining the dots what you meant by line of best fit? If not then yeiks.

I think I put down 1320 KJ /mol for 1 - propanol
 

La Divina

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oh god! i had no idea for this question

but i plotted same 200 for vertical and 10 for horizontal. i had a line of best fit.
estimated combustion was 1360 kj/mol
and i had a really stupid answer for the last part >< "cuz the propanol had impurities in it" bah!! i couldn't think of anything!!
 

amyg238

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I did a line of best fit, but it only went through the middle point, and the other two were above the line and below.

What difference is there if you do crosses or dots? I've never even thought of doing crosses in a graph...
 

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The values deviated so much a line of best fit couldn't cross through them but I drew one anyway because it is an essential scientific skill.
The point of a line of best fit is to estimate the true values obtained from an investigation.

La Divina said:
oh god! i had no idea for this question

but i plotted same 200 for vertical and 10 for horizontal. i had a line of best fit.
estimated combustion was 1360 kj/mol
and i had a really stupid answer for the last part >< "cuz the propanol had impurities in it" bah!! i couldn't think of anything!!
1360 kJ/mol is the theoretical value of the heat of combustion of ethanol.

EDIT:
Many people wrote 'impurities' for their response.
There has been an overload of Chemical Monitoring & Management questions in this paper !

amyg238 said:
I did a line of best fit, but it only went through the middle point, and the other two were above the line and below.

What difference is there if you do crosses or dots? I've never even thought of doing crosses in a graph...
SuccessOne used crosses so I used them, and they're slightly easier to see with the wandering eye ... I think.
 
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Shrikar said:
Was joining the dots what you meant by line of best fit? If not then yeiks.

I think I put down 1320 KJ /mol for 1 - propanol
Yep I got the exact same. 1320. High five!
 

La Divina

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what were ur reasons to the difference in values?
 

Armenikum

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La Divina said:
oh god! i had no idea for this question

but i plotted same 200 for vertical and 10 for horizontal. i had a line of best fit.
estimated combustion was 1360 kj/mol
and i had a really stupid answer for the last part >< "cuz the propanol had impurities in it" bah!! i couldn't think of anything!!
That still works :)
 

dwatt

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I am very worried about this question. I plotted it as dots, then did a line of best fit. increments of 40kJ/mol on y axis.

But they were all over the shot. My line of best fit was just above the first two values and below the final value. Estimated propanol at 1260 k?mol.

But then I hear of other people who obtained a perfect straight line, while others like myself had a crazy line of best fit.

Don't know what to think.
 

Fish Sauce

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La Divina said:
oh god! i had no idea for this question

but i plotted same 200 for vertical and 10 for horizontal. i had a line of best fit.
estimated combustion was 1360 kj/mol
and i had a really stupid answer for the last part >< "cuz the propanol had impurities in it" bah!! i couldn't think of anything!!
And I put impure alkanols as my reason, high five!
 

mitchbazza

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Well i drew a line of best fit, but didn't start from 0 as an alkanol can't have a molecular weight of 0. Seeing as Methanol is the smallest alkanol i started from there and the points fitted perfectly.
 

La Divina

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Fish Sauce said:
And I put impure alkanols as my reason, high five!
haha high 5! haha i feel better when other ppl have the same answers to me =) it gives a sense of secuirty..however false it may be :p
 

Shrikar

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Fish Sauce said:
Yep I got the exact same. 1320. High five!
High five indeed. So does connecting the dots the same as the line of best fit? Lol, i have no idea.
 

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The markers will get a wide range of responses especially with estimation questions.

EDIT: Thanks to all those who voted, I was getting suspicious when friends from another school used curves of best fit on their plot.

mitchbazza said:
Well i drew a line of best fit, but didn't start from 0 as an alkanol can't have a molecular weight of 0. Seeing as Methanol is the smallest alkanol i started from there and the points fitted perfectly.
But I saw it as no alkanol = no molar heat of combustion so that's why I stared from the origin,
I think that CnH2n+1OH where n > 0 explains why my plots don't fit ...
 

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Shrikar said:
High five indeed. So does connecting the dots the same as the line of best fit? Lol, i have no idea.
I just joined them because a straight line connected them pretty much perfectly on my graph.
 

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