Chemistry

posted by Natasha

In a laboratory activity, a student team measures the heat released by burning heptane. Using the following data, calculate the molar heat of combustion of heptane in kJ/mol:

Mass of water - 179.2 g
Initial water temperature - 11.6 degrees C
Final water temperature - 46.1 degrees C
Mass of heptane burned .585 g

Ok so the heat would be
(179.2)(34.5)(4.18) = 25842.43 J = 25.84 kJ, right?

And then the heat of combustion would be 25.84/44.17 = 44.17, right?

So what do I do after this? Is what I did even right?

1. DrBob222

25.84 kJ is correct but from there on no.
44.17 looks like the molar mass of CO2 but I don't know why you need that (and it may be something else). You want kJ/mol HEPTANE (isn't that C7H16).
25.84 kJ for 0.585 g sample so
25.84 kJ/0.585 gives kJ/gram and that times molar mass heptane give kJ/mol heptane.

2. Natasha

Ok I messed it up when I wrote it, it should be 25.84/.585 = 44.17
So then would it be
44.17(100.23) = 4427.16 Is that right?

The chart in my text says that the accepted molar mass of heptane is 4817. Is my answer too far away from the accepted value? Did I do it right?

3. DrBob222

It looks ok to me. I don't know what kind of accuracy you get in this type experiment but your error doesn't appear all that bad.
%error = [(4817-4427)/4817]*100 = 8.1%

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