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Thermodynamics/Energy and Phase Changes


A beaker containing 34g of water was heated to boiling. It was determined that 82.9kJ of energy was needed to completely vaporize the water. What was the initial temperature of the water?

Molar mass of H20 = 18.015g/mol

18.015g -> 1 mol
34g -> x
x = 1.9 mol

ΔT = Tf - Ti
  = 100C - Ti

Q = mcΔT
 = (34g)(4.184J/gC)(100C-Ti)

This is where I'm stuck and I don't know what to do next. If I am doing the correct steps so far, then I feel like I need to obtain the value to put in for "Q", but I don't see how to get it and also the next steps to get the actual answer.

Hi K,

The answer is in two parts because of the phase change (liquid, water to gas, steam) So calculate the heat needed to boil 34g of water. Subtract that from the total used  and you then have the heat needed to take the mass from Ti to 100c.

Hope this helps.

Best wishes,


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Thermodynamics, chemistry, chemical reactions, kinetics, chemical reaction safety, dust explosion technology, static electricity. General science.


Over 40 years experience as a practicing thermochemist in industry. Head of the fire and explosion laboratory of a major European chemical company (Ciba-Geigy). Now retired.

Institute of Chemical Engineers. Royal Society of Chemistry.

Chartered Chemist, Member of the Royal Society of Chemistry (C.Chem MRSC). Msc Sheffield.

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