A mass 20.0g of petrol was burnt in air. The heat produced was used to heat 2.5 litres of water. Given that the heat value of petrol is 43640 KJ/Kg, what was the temperature change in water?

Answer 1

The major problem with the problem is that petrol is not a pure substance; i.e., it is a mixture of several hydocarbons. Most problems of this type ASSUME that we are dealing with octane; i.e., C8H18 and I will assume that too in order to work the problem.
2C8H18 + 25O2 --> 16CO2 + 18H2O
molar mass C8H18 = 114
The problem gives 43,460 J/g of heat produced, therefore, 20 g of C8H18 will produce how much heat? That will be 43640 J/g x 20 g/2(114) = ? and I gt approximately 3800 J but you should use a more accurate value. How much will that change the temperature of the 2.5 L H2O. We know the specific heat H2O is 4.18 J/g*C so
q = mass x specific heat H2O x delta T
Substitute 3800 (use your more accurate number) = 2,500 g x 4.18 J/g*C x delta T and solve for delta T.
Post your work if you get stuck.

Answer 2

oops. That's 43640 J/MOL of the reaction and the rxn is for two moles so heat produced will be twice what I had.
2*43640 J/g x 20 g/2(114) = ? and I get approximately 7600 J. Use that new upgraded number you get for q = mass H2O x specific heat H2O x delta T. Sorry about that.

Answer 3

I want to know how can solution a temperature of change in water