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November 27, 2014

November 27, 2014

Posted by **David** on Saturday, August 5, 2006 at 8:21pm.

I have to figure out the: prediction, Analysis, and evaluation.

Problem: what is the molar enthalpy of ice?

Evidence:

mass of calorimeter: 3.76g

mass of calorimeter+ water: 103.26g

mass of calorimeter+ water+melted ice: 120.59g

initial temperature of ice:0.0 degress celcius

initial temperature of water:

32.4 degress celcius

final temp. of water: 15.6 degress celcius

The one thing I dont get is how to do the prediction and analysis. Can you help me figure out what the molar enthalpy of fusion of ice is? Thanks!

I assume you can calculate the mass of ice and mass of H2O from the data.

(massofice*deltaHfusion)+[massof ice*specificheatmeltedice*(Tf-Ti)]+[massofwater*specificheatwater*(Tf-Ti)]=0

Plug all those values in and solve for delta H fusion. If you have trouble, post your work to the point where you are stuck and we can help you through.

I didn't define my terms. Tf is the final temperature and Ti is the initial temperature of the appropriate material (meltice ice or water).

I got the answer -338.79J/mol, would that be right? And are the units right?

(17.33*deltaHfusion) + (17.33*4.19*(15.6-0)) + (99.5*4.19*-16.8) = 0

(17.33*deltaHfusion) + (-5871.2458)=0

(-5871.2458)/ (17.33)= deltaHfusion

-338.79J/mol =deltaHfusion

Thanks*

You have used 4.19 for the specific heat of water which has the units of J/g*C. The problem asks for the units per mole, not per gram. You may use your answer and multiply by the molar mass of H2O OR go back and substitute (I think it is 75.3 J/mol) for the 4.19 you used. I think I obtained about 6.08 kJ/mol or so and that is about what you have, too. My references list 4.184 and not 4.19 for the specific heat of water. At the least, I would use 4.18 and recalculate (then multiply by 18) or use 4.18 x 18.0 and get the answer in the units requested. I hope this helps.

I just looked at the problem you worked again and I think you have a sign wrong; i.e., you have 17.33X +(-5800something) = 0. When the 5800 something is carried to the other side it becomes a + number and the answer should be a positive number. And you added heat to the ice to melt it. At any rate, check your arithmetic and see if it doesn't come out a positive number. I meant to write that the first time and it slipped my mind.

THANK YOU! If I have any more questions I will ask.

oh i jus got it sry i am only in 8th..

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