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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Zoloft on February 16, 2009, 10:07:45 PM

Title: Simple Phase Change Problem (trivial error)
Post by: Zoloft on February 16, 2009, 10:07:45 PM

I think I'm making a silly mistake somewhere along my calculations, since I'm not sure why I'm off by about 300 joules, but anyway.

Calculate the total heat (in J) needed to convert 22.00 g of ice at -6.00 C to liquid water at 0.500 C

Given: mp at 1atm = 0.0 C
:delta: H_fus = 6.02 kJ/mol = 6020 J/mol
C_liquid= 4.21 J/g*C
C_solid=2.09 J/g*C

SOOO, I first calculate the energy by
H₂O (s) [-6.00C]  :rarrow: H₂O (s) [0.00C]
q=nC_{H2O Solid} :delta: T
q=(22.00gH₂O x 1 mol H₂O/ 17.02 g)(2.09 J/g*C)(0.00 C - -6.00 C)
q=15.29 J

Then,
H₂O (s) [0.00 C]  :rarrow: H₂O (l) [0.00 C]
q=n(:delta: H _fus)
q=1.22 (6.02 kJ/mol x 10³J /1 kJ)
q=7344 J

Next,
H₂O (l) [0.00 C] :rarrow: H₂O (l) [0.500]
q=nC_{H2O liquid} :delta: T
q=(22.00gH₂O x 1 mol H₂O/ 17.02 g)(4.21 J/g*C)(0.500 - 0.00)
q=2.56 J

Finally,
Q_total = 2.56 J +7344 J + 15.29 J = 7361.85 J

But the BOOK SAYS 7670 J

What the heck?!

Any help is appreciated I spent so much time on the formatting alone haha, it might be an arithmetic error or a conceptual error.  ???

Title: Re: Simple Phase Change Problem (trivial error)
Post by: macman104 on February 16, 2009, 10:26:02 PM

Water weighs 18.02 grams/mole, not 17.02.

Title: Re: Simple Phase Change Problem (trivial error)
Post by: Zoloft on February 16, 2009, 10:40:25 PM

Quote from: macman104 on February 16, 2009, 10:26:02 PM

Water weighs 18.02 grams/mole, not 17.02.

AHHHHHHHHHH  >:(

Thanks.  ;D