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Homework Help: Tomorrow i have a test, I need help with these questions

  1. May 11, 2010 #1
    Tomorrow is my test and i understand partially everything... and than i stumbled upon these two questions and if i just know what's used for each formula .. i can do it

    1) Calculate the amount of energy required to evaporate an ice cube having a mass of 45g.

    I know that first it must go for solid to liquid which is E=mf and than from liquid to gas which is E=mLv .. can someone please show me how to do them... my test is tomorrow:(

    Lf water= 3.3x10^5 J/Kg
    water=4.2x10^3 J/Kg'C
    ice= 2.1x 10^3 J/Kg'C

    the other question i didnt get was:

    2)An 80g mandarin orange is burned in bomb calorimeter. The 1.55 kg of water change in temperature from 21.0'C to 39.4'C. Calculate the energy per gram in the mandarin orange.

    I would show you my attempt except its just ridiculous becuase i reallyyyy dont get it.

    Your help is muchhhh appreciated.. i need help ASAP ...thank you
  2. jcsd
  3. May 11, 2010 #2


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    Homework Helper

    Well for the first one, you will need energy to

    1)convert the ice to liquid which is (solid to liquid)
    2) Raise the temperature to the boiling point of water
    3) convert all the liquid to vapor

    So find the individual energy requirements and add them up.

    For the second one, how much energy would it take to raise the temperature of water of mass 1.55kg from 21C ro 39.4C?.
  4. May 11, 2010 #3
    Thats the problem... i dont know how to convert solid to liquid.. what formula do i use?
  5. May 11, 2010 #4
    Q=mC C is a constant you can look up for water its different depending on which phase transition you are calculating.
    Q=mc(Tf-Ti) for when your raising the temperature c i think its 1cal/gC for water.

    Add all the Qs and viola
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