# Energy needed for car to travel-thermodynamics

1. Apr 12, 2010

### aal0315

1. The problem statement, all variables and given/known data
Gasoline yields 4.8x10^7 J/kg when burned. How much energy does a car use travelling a distance of 150 km if it gets 25 miles per (British) gallon? Assume that the density of gasoline is 68% that of water.

2. Relevant equations

3. The attempt at a solution
I dont really know where to begin with this question. All i know is that 25mpg is equal to 40.225km per gallon.
Any suggestions?

2. Apr 12, 2010

The crux of this question really is in unit conversion. It assumes that you know that a litre of water has a mass of 1 kg so that you can work out the mass of 25 gallons of gasoline.

Does this help?

3. Apr 12, 2010

### aal0315

So i do 40.225 x 68% = 27.35 kg to get the mass of 25 gallons of gasoline?

and what next? I dont know a formula to figure out the energy given the information.

4. Apr 14, 2010

### aal0315

ok does this look right to anyone:
25gallons x 1.609 = 40.225km per gallon

150km/40.225 = 3.729 gallons to travel 150km

3.729 x 68% = 2.536kg

2.536 x 4.8x10^7J/kg = 1.22x10^9J ???

5. Apr 24, 2010

### Unit Maestro

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Last edited by a moderator: May 4, 2017