# Homework Help: Another problem (Phase Change Heat Calculation)

1. Dec 12, 2017

### Lori

1. The problem statement, all variables and given/known data

2. Relevant equations
q = mcat
mols*hvap

3. The attempt at a solution

To decrease temp from 70 to 56 degrees (bp), q=mcat = 1195.5
to convert this gas into liquid , must do mols * hvap = 29129 joules
to decrease temp from 56 to 0 thought, i get q= mcat = -7047 J (which is negative- is this wrong?)

it seems like the correct answer is when 7047 is positive after adding these up,. i get 37.37 kj. which seams like the answer (cant see it cause my professor erased it...)

Why wouldn't i subtract the last number?

2. Dec 13, 2017

### Staff: Mentor

No idea where you got the minus from - cooling the liquid is no different from cooling the gas (for which you got the correct answer).

3. Dec 13, 2017

### Lori

from mcat , i got the negative from change in temperature (final mius initial) so (0-56)

4. Dec 13, 2017

### Staff: Mentor

So, for colling of liquid ΔT = final - initial = 0 - 56 = -56, yes?

And what was the change in temperature for the cooling of gas?

5. Dec 13, 2017

### Lori

Oh I see. We are looking for any heat removed from cooling the gas so we should add heat to cool the liquid since it was used to cool the gas as well