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    I'm really stuck with a question from the AS Unit 2 June 2010 paper.

    "The student showed that the temperature of 200 g water increased by 8 degrees c when 0.46 g of pure ethanol was burned in air and the heat was used to warm water.

    "Use these results to calculate the value in KJmol-1 obtained by the student for this enthalpy change."

    So I did Q = M x c x delta T which gave me 6688

    Then I went to do Q / moles

    Which I thought would be 6688 divided by 0.01 .

    However the answer on the mark scheme is -668.8

    So I'm really confused. Probably a stupid mistake I can't see.

    I'm terrible at chemistry, so thanks for the help!
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    (Original post by NadezhdaK)
    I'm really stuck with a question from the AS Unit 2 June 2010 paper.

    "The student showed that the temperature of 200 g water increased by 8 degrees c when 0.46 g of pure ethanol was burned in air and the heat was used to warm water.

    "Use these results to calculate the value in KJmol-1 obtained by the student for this enthalpy change."

    So I did Q = M x c x delta T which gave me 6688

    Then I went to do Q / moles

    Which I thought would be 6688 divided by 0.01 .

    However the answer on the mark scheme is -668.8

    So I'm really confused. Probably a stupid mistake I can't see.

    I'm terrible at chemistry, so thanks for the help!
    If you use the formula with mass in grams then your energy is given in Joules. You must then divide by 1000 to convert to kJ ...

    Also, as it is an exothermic reaction the answer must be expressed as a negative value.
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    Oh yes, thank you!! Knew it would be a stupid mistake...
 
 
 
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