[lou_skywalker]

Pure liquid octane (C8H18; d = 0.702 g/mL) is used as the fuel in a test of a new automobile drive train.

(a) How much energy (in kJ) is released by complete combustion of the octane in a 27.0 gal fuel tank to gases (ΔH°combined = -5.45E3 kJ/mol)?

(b) The energy delivered to the wheels at 65 mph is 5.5E4 kJ/h. Assuming all the energy is transferred to the wheels, what is the cruising range (in km) of the car on a full tank?

I got -3.58e6 for part a but stupid webassign says its within 10% of the correct value...

[sjb]

Pure liquid octane (C8H18; d = 0.702 g/mL) is used as the fuel in a test of a new automobile drive train.

(a) How much energy (in kJ) is released by complete combustion of the octane in a 27.0 gal fuel tank to gases (ΔH°combined = -5.45E3 kJ/mol)?

(b) The energy delivered to the wheels at 65 mph is 5.5E4 kJ/h. Assuming all the energy is transferred to the wheels, what is the cruising range (in km) of the car on a full tank?

I got -3.58e6 for part a but stupid webassign says its within 10% of the correct value...

What's your working, and whereabouts are you? I could lead you one way, but it may be wrong due to your location.

[lou_skywalker]

Pure liquid octane (C8H18; d = 0.702 g/mL) is used as the fuel in a test of a new automobile drive train.

(a) How much energy (in kJ) is released by complete combustion of the octane in a 27.0 gal fuel tank to gases (ΔH°combined = -5.45E3 kJ/mol)?

(b) The energy delivered to the wheels at 65 mph is 5.5E4 kJ/h. Assuming all the energy is transferred to the wheels, what is the cruising range (in km) of the car on a full tank?

I got -3.58e6 for part a but stupid webassign says its within 10% of the correct value...

ok so I converted 27 gallons to mL and multipled it by 0.702 to get the number of moles of octane combusted (which is 628 moles)
then using the reaction 2C8H18 + 25O2 ---> 16CO2 + 18H2O to find the heat of formation of the reactant side, and added to the total heat change to give me ΔH°product.

What's your working, and whereabouts are you? I could lead you one way, but it may be wrong due to your location.

[sjb]

How many mls are there in your gallon? To me there are 4546, making 122744.481 ml for 27 gallons but I'm not sure if that is right for you. This makes 755.848 mol of octane...

[lou_skywalker]

according to google convert 1 US gallon = 3 785.41178 mL, so 27 gallons is 102 206.118 mL

[sjb]

according to google convert 1 US gallon = 3 785.41178 mL, so 27 gallons is 102 206.118 mL

Ahh, so we're talking different gallons - that's why I wanted to know where you are

OK, so there are 102 206.118 ml of octane, mass 71.748 kg, moles 629.37 (so we're in the right area here), total energy released 3430000 kJ, or 3.43 x 10⁶ kJ.

Perhaps you are rounding too prematurely in your intermediate answers (volume, mass, moles) ?

[lou_skywalker]

hmm yeah maybe I rounded off too early, or maybe my "CPU" was overheated at that time.
Anyways got it now many thanks!