# How do u know if u need to use mass of water or alcohol - enthalpy change a level

How do u know if u need to use mass of water or alcohol - enthalpy change a level
(edited 3 weeks ago)
For standard combustion calorimetry questions like this, always use the mass of water (since it's what's being heated).
Step 1 uses mass of water as in this experiment, you're measuring the temperature of the water caused by energy released from alcohol combustion. Alcohol provides the energy, you don't measure the temperature of that alcohol. When it comes to enthalpy calculations using q= mass x specific heat capacity x temperature change, it's what is being heated that you use the mass of eg. usually some solution in insulated container such as a polystyrene cup or in this case, the calorimeter
Original post by Rohan007best
How do u know if u need to use mass of water or alcohol - enthalpy change a level

For the calculation of heat energy using Q = mcΔθ, always use the mass of water.

To calculate the moles of the substance burnt, use the mass of that substance and the formula n = m/Mr, where n = moles, m = mass of substance burnt and Mr = relative formula mass of substance burnt.
Original post by Rohan007best
How do u know if u need to use mass of water or alcohol - enthalpy change a level

Hi, so basically the way I like to think of I is that since we are calculating the enthalpy change of combustion we know that heat is being transferred from the chemical system (alcohol) to the surroundings, and in this case the surroundings is the water, therefore we use the mass of the water, specific heat capacity of the water and the temp change of the water