Enthalpy Change of Combustion.

jinhuit95

1. The problem statement, all variables and given/known data
When 1.00g of ethanol was burned under a container of water, it was found that 100g of water was heated from, 15°C to 65°C, The process was known to be only 70% efficient. Calculate the enthalpy change of combustion of ethanol taking specific heat capacity of water to be 4.18 J g^-1 k^-1.


2. Relevant equations
q= mc ΔT; ΔHr= - q/ moles of limiting reagent.


3. The attempt at a solution
I calculate q = 100 x 4.18 x 50 x(70/100) = 14630J.
Then I found the number of moles of ethanol which is 1/46 = 0.0217
Therefore enthalpy change will then be -14630/0.0217 = -672980J = -673 kJ mol^-1 but the answer given is -1.37 x 10^3 kJ mol^-1. Where did I go wrong??

LawrenceC

In your first step you should be dividing by 70% rather than multiplying by 70%.