# AS chemistry, please help

Hi, I'm stuck on these questions, if you know any of the answers please help me by showing the method, thank you!
1) calculate the amount of energy released by burning 45 litres of petrol, use the following info to help - assume that petrol is octane (C8H18), the standard enthalpy change of combustion is -5500KJ/mol, the density of octane is 0.70 g/cm3 and 1 litre is 1000cm3

2) calculate the mass and volume of hydrogen needed to provide the same amount of energy as 45 litres of octane - the standard enthalpy change of combustion of H2 is -286 KJ/mol and 1 mole of gas at 20°c and atmospheric pressure has a volume of about 24 litres.

3) H2 engines are 20% more efficient than petrol on the motorway and 50% more efficient than petrol in cities:
What mass and volume of H2 are needed to give the same mileage as 45 litres of petrol in
i) motorway
ii) city?
Thank you for any help!

Posted Wed 2nd January, 2013 @ 18:07 by K.N

## 3 Answers

1) first convert litres to cm3, so 45 litres = 45,000 cm3

Density = mass / volume, but we want to calculate mass so rearrange to get mass = density x volume

So mass of octane is 0.70 x 45,000 = 31500g

We want to find the number of moles of octane using moles = mass / Mr

Mr = 16 x 8 + 1 x 18 = 146

Therefore, moles of octane = 31500 / 146 = 215.75mol

The standard enthalpy change of combustion is then 1 mole is completely combusted in excess oxygen, so the enthalpy change when 215.75 moles are combusted is 215.75 x -5500 = -1190000 KJ to 3 significant figures. Therefore the energy released is 1190000 KJ

Answered Sat 5th January, 2013 @ 12:23 by Ross Edwards

2) to avoid rounding errors we'll use a more accurate value of the energy released when 45 litres of octane are combusted which is 1186644 KJ

If one mole of H2 combusts to release 286 KJ of energy, then 1186644/286 moles are needed to release 1186644 KJ of energy. This is 4149 moles of H2.

Mass = moles x Mr so mass of H2 = 4149 x 2 (as Mr of H2 is 2.00) = 8300g to 3 significant figures

If 1 mole of H2 occupies 24 litres, then 4149 moles of H2 occupies 4149 x 24 = 99600 litres to 3 significant figures

Answered Sat 5th January, 2013 @ 12:30 by Ross Edwards

Thank you

Answered Wed 9th January, 2013 @ 16:48 by K.N