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Heat that needed

  1. Nov 7, 2007 #1
    Heat that needed(q10)

    For this question:
    http://tinyurl.com/2leprw

    Followed is my working

    Q=m_w*c_w*deltaT + m_w*L_f + m_w*c_i*deltaT
    =m_w[
    c_w*deltaT +
    L_f +
    c_i*deltaT
    ]
    =2*(10^3)g [
    1 cal/(g*celcius) * 20 celcius +
    (3.33*(10^5)*0.2389/1000)calories/g +
    0.5 cal/(g*celcius) * 10 celcius
    ]
    =209100J

    And this is not one of the choices...
    Can anybody kindly pointed out what is my mistake?
     
    Last edited: Nov 7, 2007
  2. jcsd
  3. Nov 8, 2007 #2

    Astronuc

    User Avatar

    Staff: Mentor

    Well the choice of answers are in kcal rather than J.

    Q=m_w*c_w*deltaT + m_w*L_f + m_w*c_i*deltaT

    is correct. M = 2000 g (2 kg) so

    Q = 2000 g ( c_w*c_w*deltaT + L_f + c_i*deltaT)

    = 2000 g ( 1 cal/(g-°C) * 20°C + 79.7 cal/g + 0.5 (cal/g°C) * 10°C)

    Be careful with conversions, and please show units.
     
  4. Nov 8, 2007 #3

    Kurdt

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Using the values you've given even I don't get an answer thats on the sheet.
     
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