A coffee-cup calorimeter initially contains 125 g water at 24.2 degrees celsius. Potassium bromide (10.5 g), also at 24.2 degree celsius, is added to the water, and after the KBr dissolves, the final temperature is 21.1 degrees celsius. Calculate the enthalpy change for dieseling the salt in J/g and kJ/mol. Assume that the specific heat capacity of the solution is 4.18 J/Cg and that no heat is transferred to the surrounds or to the calorimeter.

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  1. q = heat released by the water = mass H2O x specific heat H2O x (Tfinal-Tinitial).
    Then q/gram KBr = q/10.5 = dH J/g.
    dH J/mol = J/gram KBr x (molar mass KBr/1 mol KBr)
    Then J/mol x (1 kJ/1000 J) = kJ/mol

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  2. Don't forget that this is heat released by the H2O (it gets colder) so the KBr is absorbing q.

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