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! 1. The problem statement, all variables and given/known data Now I'm on thermodynamics :) A mass m1 = 150g of water ice of temperature T1 = -10C is put in an isolated container, which already contains a mass m2 of water vapor of temperature T2 = 120C. After a while there is only liquid water in the container at a temperature T3 = 40C. What was the mass m2 of the water vapor? Fusion heat of ice: Λs = 333kJ/kg Vaporization heat of water: Λv = 2256 kJ/kg Specific heat of water ice: cE = 2.06 kJ/(kg⋅K) Specific heat of water: cW = 4.19 kJ/(kg⋅K) Specific heat of water vapor: cD = 2.08 kJ/(kg⋅K) 2. Relevant equations Q = m⋅c⋅ΔT Qchange of state = m⋅Λ 3. 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 Q1 and Q2 are the heat needed respectively for m1 and m2 to reach T3, then Q1 = -Q2. If I take the absolute value, that gives me: m1⋅(10⋅cE + Λs + 40⋅cW) = m2⋅(-20⋅cD + Λv - 60⋅cW) 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 m2 ≈ 40g with the minus, and m2 ≈ 30g with the plus. Hard for me to say which is right. Thanks a lot in advance for your answers. Julien.