Thermodyanmics, thermal properties of matter

  • #1

Homework Statement


a beaker contains 200mL of ethyl alcohol at 20degrees. what is the minumum amount of energy that must be removed to produce solid ethyl alcohol..

the melting/freezing pt is 114degrees Lf=1.09x10^5
density is 790kg/m^3
specific heat cap = 2400J/kgK

Homework Equations


Q=mcdT
Q=ML
m=pv

The Attempt at a Solution


m=pv=(2.00*10^-4m^3)(790)=0.158kg
Q1=mcdT
=0.158(2400)(94)
=35644.8J
Q2=ML
=0.158(1.09*10^5)
=17222J

Qtotal=Q1+Q2
=52866.8J

i think i did somethign wrong because in the book it says the answer is 68000J can someone help me out i dont know what i did wrong?
 

Answers and Replies

  • #2
Redbelly98
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Homework Statement


a beaker contains 200mL of ethyl alcohol at 20degrees. what is the minumum amount of energy that must be removed to produce solid ethyl alcohol..

the melting/freezing pt is 114degrees
That 114 degrees must be wrong. If it were true, it would already be a solid at 20 degrees.

Lf=1.09x10^5
density is 790kg/m^3
specific heat cap = 2400J/kgK

Homework Equations


Q=mcdT
Q=ML
m=pv

The Attempt at a Solution


m=pv=(2.00*10^-4m^3)(790)=0.158kg
Q1=mcdT
=0.158(2400)(94)
=35644.8J
Q2=ML
=0.158(1.09*10^5)
=17222J

Qtotal=Q1+Q2
=52866.8J

i think i did somethign wrong because in the book it says the answer is 68000J can someone help me out i dont know what i did wrong?
 
  • #3
That 114 degrees must be wrong. If it were true, it would already be a solid at 20 degrees.

oooh sorry that 114 is suppose to be -114
 
  • #5
Okay, then do you see the error here:
oo i added it wrong for dT its suppose to be -114-20 instead of 114-20

ok thank you
 

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