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Math Help - Enthalpy change

  1. #1
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    Enthalpy change

    Hello, got this question that didn't have the answer to it in the back of the textbook, so I just wanna know if I'm right. :-)

    "In a laboratory experiment, 1.16g of an organic liquid fuel was completely burned in oxygen. The heat formed during this combustion raised the temperature of 100g of water from 295.3K to 3578K. Calculate the standard enthalpy of combustion, delta H, of fuel. It's Mr is 58"

    So my working was:

    heat transferred = mc(delta t) = 100 x 4.2 x 62.5 = 26250

    no. of moles = mass / mr = 1.16 / 58 = 0.02

    delta H = heat transferred / no. of moles
    = 26250 / 0.02
    = 1312500 kJ / mol

    Is this correct? Thank you.
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  2. #2
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    Re: Enthalpy change

    Quote Originally Posted by yorkey View Post
    Hello, got this question that didn't have the answer to it in the back of the textbook, so I just wanna know if I'm right. :-)

    "In a laboratory experiment, 1.16g of an organic liquid fuel was completely burned in oxygen. The heat formed during this combustion raised the temperature of 100g of water from 295.3K to 3578K. Calculate the standard enthalpy of combustion, delta H, of fuel. It's Mr is 58"

    So my working was:

    heat transferred = mc(delta t) = 100 x 4.2 x 62.5 = 26250

    no. of moles = mass / mr = 1.16 / 58 = 0.02

    delta H = heat transferred / no. of moles
    = 26250 / 0.02
    = 1312500 kJ / mol

    Is this correct? Thank you.

    If the temp change was 295.3K to 3578K
    then delta T won't be 62.5K
    >>

    You forgot to change the mass of water to kg
    and the ans that you will get should be -ve (opposite to the amount of heat energy that is taken is by water)

    So the final ans should be -1312.5 KJ/mol (assuming that delta t is 62.5K)
    Thanks from topsquark
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