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?
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.
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?
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%
Yes, your approach is correct so far. You correctly calculated the heat released by the burning of heptane using the formula:
Heat = mass of water × change in temperature × specific heat capacity of water
Using the given values, the heat released is:
Heat = 179.2 g × (46.1 °C - 11.6 °C) × 4.18 J/g°C = 25842.43 J = 25.84 kJ
Next, you need to calculate the molar heat of combustion of heptane. To do this, you need to know the molar mass of heptane, which is 100.21 g/mol.
Now, divide the heat released (25.84 kJ) by the mass of heptane burned (0.585 g) to find the molar heat of combustion:
Molar heat of combustion = Heat released / Mass of heptane burned
Molar heat of combustion = 25.84 kJ / 0.585 g
Molar heat of combustion ≈ 44.17 kJ/mol
Therefore, the molar heat of combustion of heptane is approximately 44.17 kJ/mol.