Is this a valid method to find percentage of pure ethanol in an impure sample? To generalize the problem, I will just say:


  5 gram of impure ethanol (or any other type of combustible material) is placed inside a bomb calorimeter where volume is constant. If the combustion produces a certain amount of energy, how should I find the mass in gram of pure ethanol contained in that quantity of impure ethanol?


Here is what I will do:


find the standard molar enthalpy of ethanol. 
write a combustion equation. 
find the energy produced in the combustion of one mole of ethanol. 
then divide that amount of ethanol by the amount of energy stated in problem. 


To find how many mole of pure ethanol, convert the mole into gram.

But there are two things that make me think that this way of solving the problem is not correct:


the quantity of impure ethanol is a mixture of pure ethanol and another unknown material. What if this unknown material produce part of the energy stated in the problem?
the standard molar enthalpy is measure under standard conditions (1 atm, 25 °C). However, the condition inside the bomb calorimeter is unknown.
What if this unknown material produce part of the energy stated in the problem?


It would be stated in the problem that the amount of heat that the unknown material produced.


  The standard molar enthalpy is measured under standard conditions (1 atm, 25°C). However, the condition inside the bomb calorimeter is unknown.


We are given some information, the final temperature of the system. But you are right in that we do not know the pressure, and the specific heat of the remaining items is difficult to calculate.