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Hi everybody! To begin I'd like to say a big thanks to everyone who helped me on the forum lately: I passed my exams recently and without your help I'm not sure I could have done it!

Now I'm on thermodynamics :)

A mass m

Fusion heat of ice: Λ

Vaporization heat of water: Λ

Specific heat of water ice: c

Specific heat of water: c

Specific heat of water vapor: c

Q = m⋅c⋅ΔT

Q

I think I'm pretty much able to do this, but I do have a question regarding the signs in the equations.

If Q

m

Is that correct? Another student did the same but took the absolute value of each change of temperature (so had + instead of the two - on the right side of the equation), and we couldn't really find out yet what's right or wrong. Could someone please help us? :)

Btw I come up with m

Thanks a lot in advance for your answers.

Julien.

1. Homework Statement1. Homework Statement

Now I'm on thermodynamics :)

A mass m

_{1}= 150g of water ice of temperature T_{1}= -10C is put in an isolated container, which already contains a mass m_{2}of water vapor of temperature T_{2}= 120C. After a while there is only liquid water in the container at a temperature T_{3}= 40C. What was the mass m_{2}of the water vapor?Fusion heat of ice: Λ

_{s}= 333kJ/kgVaporization heat of water: Λ

_{v}= 2256 kJ/kgSpecific heat of water ice: c

_{E}= 2.06 kJ/(kg⋅K)Specific heat of water: c

_{W}= 4.19 kJ/(kg⋅K)Specific heat of water vapor: c

_{D}= 2.08 kJ/(kg⋅K)## Homework Equations

Q = m⋅c⋅ΔT

Q

_{change of state}= m⋅Λ## The Attempt at a Solution

I think I'm pretty much able to do this, but I do have a question regarding the signs in the equations.

If Q

_{1}and Q_{2}are the heat needed respectively for m_{1}and m_{2}to reach T_{3}, then Q_{1}= -Q_{2}. If I take the absolute value, that gives me:m

_{1}⋅(10⋅c_{E}+ Λ_{s}+ 40⋅c_{W}) = m_{2}⋅(-20⋅c_{D}+ Λ_{v}- 60⋅c_{W})Is that correct? Another student did the same but took the absolute value of each change of temperature (so had + instead of the two - on the right side of the equation), and we couldn't really find out yet what's right or wrong. Could someone please help us? :)

Btw I come up with m

_{2}≈ 40g with the minus, and m_{2}≈ 30g with the plus. Hard for me to say which is right.Thanks a lot in advance for your answers.

Julien.

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