Find the Mass of Water that Freezes: Entropy Change Calculations

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Homework Help Overview

The discussion revolves around a thermodynamics problem involving the mixing of water and ice, specifically focusing on the mass of water that freezes and the change in entropy of the system. The scenario includes specific heat capacities and the heat of fusion for water.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts to calculate the heat exchanges between the ice and water, questioning their results when the sums do not balance. They explore the concept of leftover heat and its relation to the mass of water that freezes.

Discussion Status

Participants are actively engaging with the calculations presented. Some express uncertainty about the original poster's results, while one participant suggests that the original poster's calculated mass of 9.4 grams may be correct, indicating a potential discrepancy with an answer key.

Contextual Notes

There is a noted correction regarding the heat of fusion value, which may impact the calculations. The original poster also expresses confusion about their results and seeks clarification on where they might have gone wrong.

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1. At 10°C, 32 g of water is mixed with 430 g of ice at -5°C. (The heat capacity of water is 4190 J/(kg * °C), that of ice is 2090 J/ (kg *°C), and the heat of fusion of water is 3.34 * 10^5 J/kg)
a) What mass of water freezes ?
b) What is the change in entropy of the system?

What I did on part (a)

ice : -5°C -> 0°C
Q = m_ice C_ice delta T
Q = (0.43 kg) (2090 J/ (kg *°C) ) (0°C-(-5°C))
Q = + 4493.5 J

water: 10°C -> 0°C
Q = m_water C_water delta T
Q = (0.032 kg) (4190 J/ (kg *°C) ) (0°C-(10°C))
Q = - 1340.8 J

Qice + Qwater = 0
but it doesn't equal to zero.

Not sure what to do next.
Qice + Qwater + Qleftover = 0
Qleftover = m_Water Lf
m_Water = Qleftover / Lf
= 3152.7 J / (3.34 * 10 ^5 J/kg)
=0.00 9439 kg which is approximately 9.4 grams
Doesn't worked out to be the answer of 11 grams.

Where did I gone wrong?
Your help is appreciated.
Thanks.
 
Last edited:
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The heat of fusion of water is 3.34*10^3 kJ/kg.
 
typo error. 3.34 *10^5 J/kg or 334 * 10^3 J/kg
I corrected the above typo error on the 1st post.
 
Your result, 9.4 grams, looks okay to me. Could be an error in the answer key.
 

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