Heat of Combustion Lab Question.?

Question

So I have 200 mL of Water.

The mass of the calorimeter is 24.375g.

The initial mass of the calorimeter w/ the water is 217.833g (193.458g of water)

The final mass is 217.607 (193.232g of water).

The initial temperature is 23.0°C, the final temp is 43.0°C, the change is 20°C.

The mass of ethanol burnt is 1.418g.

So I need to know the Heat released from this mass of ethanol J/g

And Heat released from 1 mole of ethanol kJ/mol.

So I know I use the equation q= -mass x SHC x (change)Temp.

I'm mostly confused about what I plug in for the "mass" part. SHC is 4.184 J/K*g. And the change in Temp would be the 20.0°C which would have to be changed to Kelvins.

But if anyone could show me how to answer both questions fully, I would appreciate it ^ ^. Thanks.

Answer 1

Not that they are trying to trick you, but you need to consider the temp change in two systems: the water and the body of the calorimeter. So the equation really looks like this…

Total heat absorbed (to 3 sig figs) = (20.0 K) x (1 J/K*g) x (193.458g of water) + (20.0 K) x (4.184 J/K*g) x (24.375 of calorimeter).

The magnitude of a degree is the same in both Celsius and Kelvin, only the zero points differ for the two scales. So a change of 20.0 C, is a change of 20.0 K.

Remember also that no calorimeter is 100% efficient, a healthy portion of heat leaks away in even the best systems, so your answer should typically be understated compared to what you might find reported in literature.