self test 4A.1A

$\Delta U=q+w$

Skyllar Kuppinger 1F
Posts: 52
Joined: Thu Jul 25, 2019 12:16 am

self test 4A.1A

"Water expands when it freezes. How much work does 100.g of water do when it freezes at 0 degrees Celsius and pushes back the metal wall of a pipe that exerts an opposing pressure of 1070 atm? the densities of water and ice at 0 degrees Celsius are 1.00g/cm^3 and 0.92g/cm^3, respectively."

the answer is -0.86 kj but I keep getting -0.94. can someone tell me what I'm doing wrong?
I used work=-PdeltaV.
I used 1070 for P. for delta V I did the volume of the ice minus the volume of the water. density=mass/volume so volume = mass/density. It is 100 grams of water so volume of ice is 100/0.92 which is 109 and for liquid water it is 100/1 which is 100. so that's 109-100 which is 9. but I believe this is in mL bc one cm^3 = 1 mL. So I converted it to L: 9 mL = 0.009 L.
so work = -1070x0.009 which is 9.63. This is in Latms, there are 101.325 joules in 1 Latm so the work in joules is 9.63x101.325. then divide that by 1000 because the answer was in kJ and you get 0.94 (I kept the numbers in my calculator, I did not prematurely round).

Why am I getting the answer wrong? pls help

Dina Marchenko 2J
Posts: 54
Joined: Thu Jul 25, 2019 12:16 am

Re: self test 4A.1A

I followed your math and got the same answer. The only thing that I can guess is that maybe the book did prematurely round and keep their calculations to 4 sig figs or something. I think that your answers are close enough that you wouldn't be penalized if this were on a test.

Juliet Stephenson 4E
Posts: 100
Joined: Wed Sep 18, 2019 12:21 am

Re: self test 4A.1A

The first time I did this problem is ended up with -.86 kJ but it was because I made an error (multiplied mass times density instead of dividing). I am thinking the answer key is wrong. I don't think such a large discrepancy could be produced by premature rounding.